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GNU/Linux |
CentOS 5.6 |
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mdadm(8) |
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mdadm − manage MD devices aka Linux Software RAID
mdadm [mode] <raiddevice> [options] <component-devices>
RAID devices are virtual devices created from two or more real block devices. This allows multiple devices (typically disk drives or partitions thereof) to be combined into a single device to hold (for example) a single filesystem. Some RAID levels include redundancy and so can survive some degree of device failure.
Linux Software RAID devices are implemented through the md (Multiple Devices) device driver.
Currently, Linux supports LINEAR md devices, RAID0 (striping), RAID1 (mirroring), RAID4, RAID5, RAID6, RAID10, MULTIPATH, and FAULTY.
MULTIPATH is not a Software RAID mechanism, but does involve multiple devices: each device is a path to one common physical storage device.
FAULTY is also not true RAID, and it only involves one device. It provides a layer over a true device that can be used to inject faults.
mdadm has
several major modes of operation:
Assemble
Assemble the components of a previously created array into an active array. Components can be explicitly given or can be searched for. mdadm checks that the components do form a bona fide array, and can, on request, fiddle superblock information so as to assemble a faulty array.
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Build |
Build an array that doesn’t have per-device superblocks. For these sorts of arrays, mdadm cannot differentiate between initial creation and subsequent assembly of an array. It also cannot perform any checks that appropriate components have been requested. Because of this, the Build mode should only be used together with a complete understanding of what you are doing. | ||
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Create |
Create a new array with per-device superblocks. |
Follow or Monitor
Monitor one or more md devices and act on any state changes. This is only meaningful for raid1, 4, 5, 6, 10 or multipath arrays, as only these have interesting state. raid0 or linear never have missing, spare, or failed drives, so there is nothing to monitor.
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Grow |
Grow (or shrink) an array, or otherwise reshape it in some way. Currently supported growth options include changing the active size of component devices and changing the number of active devices in RAID levels 1/4/5/6, as well as adding or removing a write-intent bitmap. |
Incremental Assembly
Add a single device to an appropriate array. If the addition of the device makes the array runnable, the array will be started. This provides a convenient interface to a hot-plug system. As each device is detected, mdadm has a chance to include it in some array as appropriate.
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Manage |
This is for doing things to specific components of an array such as adding new spares and removing faulty devices. | ||
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Misc |
This is an ’everything else’ mode that supports operations on active arrays, operations on component devices such as erasing old superblocks, and information gathering operations. |
Auto-detect
This mode does not act on a specific device or array, but rather it requests the Linux Kernel to activate any auto-detected arrays.
−A, −−assemble
Assemble a pre-existing array.
−B, −−build
Build a legacy array without superblocks.
−C, −−create
Create a new array.
−F, −−follow, −−monitor
Select Monitor mode.
−G, −−grow
Change the size or shape of an active array.
−I, −−incremental
Add a single device into an appropriate array, and possibly start the array.
−−auto-detect
Request that the kernel starts any auto-detected arrays. This can only work if md is compiled into the kernel — not if it is a module. Arrays can be auto-detected by the kernel if all the components are in primary MS-DOS partitions with partition type FD. In-kernel autodetect is not recommended for new installations. Using mdadm to detect and assemble arrays — possibly in an initrd — is substantially more flexible and should be preferred.
If a device is given before any options, or if the first option is −−add, −−fail, or −−remove, then the MANAGE mode is assumed. Anything other than these will cause the Misc mode to be assumed.
−h, −−help
Display general help message or, after one of the above options, a mode-specific help message.
−−help−options
Display more detailed help about command line parsing and some commonly used options.
−V, −−version
Print version information for mdadm.
−v, −−verbose
Be more verbose about what is happening. This can be used twice to be extra-verbose. The extra verbosity currently only affects −−detail −−scan and −−examine −−scan.
−q, −−quiet
Avoid printing purely informative messages. With this, mdadm will be silent unless there is something really important to report.
−b, −−brief
Be less verbose. This is used with −−detail and −−examine. Using −−brief with −−verbose gives an intermediate level of verbosity.
−f, −−force
Be more forceful about certain operations. See the various modes for the exact meaning of this option in different contexts.
−c, −−config=
Specify the config file. Default is to use /etc/mdadm.conf, or if that is missing then /etc/mdadm/mdadm.conf. If the config file given is partitions then nothing will be read, but mdadm will act as though the config file contained exactly DEVICE partitions and will read /proc/partitions to find a list of devices to scan. If the word none is given for the config file, then mdadm will act as though the config file were empty.
−s, −−scan
Scan config file or /proc/mdstat for missing information. In general, this option gives mdadm permission to get any missing information (like component devices, array devices, array identities, and alert destination) from the configuration file (see previous option); one exception is MISC mode when using −−detail or −−stop, in which case −−scan says to get a list of array devices from /proc/mdstat.
−e , −−metadata=
Declare the style of superblock (raid metadata) to be used. The default is 0.90 for −−create, and to guess for other operations. The default can be overridden by setting the metadata value for the CREATE keyword in mdadm.conf.
Options are:
0, 0.90, default
Use the original 0.90 format superblock. This format limits arrays to 28 component devices and limits component devices of levels 1 and greater to 2 terabytes.
1, 1.0, 1.1, 1.2
Use the new version-1 format superblock. This has few restrictions. The different sub-versions store the superblock at different locations on the device, either at the end (for 1.0), at the start (for 1.1) or 4K from the start (for 1.2).
−−homehost=
This will override any HOMEHOST setting in the config file and provides the identity of the host which should be considered the home for any arrays.
When creating an array, the homehost will be recorded in the superblock. For version-1 superblocks, it will be prefixed to the array name. For version-0.90 superblocks, part of the SHA1 hash of the hostname will be stored in the later half of the UUID.
When reporting information about an array, any array which is tagged for the given homehost will be reported as such.
When using Auto-Assemble, only arrays tagged for the given homehost will be assembled.
−n, −−raid−devices=
Specify the number of active
devices in the array. This, plus the number of spare devices
(see below) must equal the number of
component-devices (including
"missing" devices) that are listed on the
command line for −−create. Setting a
value of 1 is probably a mistake and so requires that
−−force be specified first. A value of 1
will then be allowed for linear, multipath, raid0 and raid1.
It is never allowed for raid4 or raid5.
This number can only be changed using
−−grow for RAID1, RAID5 and RAID6 arrays,
and only on kernels which provide necessary support.
−x, −−spare−devices=
Specify the number of spare (eXtra) devices in the initial array. The number of component devices listed on the command line must equal the number of raid devices plus the number of spare devices.
After initial array creation, new devices are added to the array using the −−add command. If you add devices in excess of the number needed for the array, they are automatically treated as spare devices. For grow mode, it is not possible to grow the number of spare devices, instead you need to grow (or shrink) the number of active devices in the array. Spare devices are handled automatically after initial array creation.
−z, −−size=
Amount (in Kibibytes) of space to use from each drive in RAID level 1/4/5/6. This must be a multiple of the chunk size, and must leave about 128Kb of space at the end of the drive for the RAID superblock. If this is not specified (as it normally is not) the smallest drive (or partition) sets the size, though if there is a variance among the drives of greater than 1%, a warning is issued.
This value can be set with −−grow for RAID level 1/4/5/6. If the array was created with a size smaller than the currently active drives, the extra space can be accessed using −−grow. The size can be given as max which means to choose the largest size that fits on all current drives.
−c, −−chunk=
Specify chunk size in kibibytes. The default is 64.
−−rounding=
Specify rounding factor for linear array (==chunk size)
−l, −−level=
Set raid level. When used with −−create, options are: linear, raid0, 0, stripe, raid1, 1, mirror, raid4, 4, raid5, 5, raid6, 6, raid10, 10, multipath, mp, faulty. Obviously some of these are synonymous.
When used with −−build, only linear, stripe, raid0, 0, raid1, multipath, mp, and faulty are valid.
Not yet supported with −−grow.
−p, −−layout=
This option configures the fine details of data layout for raid5, and raid10 arrays, and controls the failure modes for faulty.
The layout of the raid5 parity block can be one of left−asymmetric, left−symmetric, right−asymmetric, right−symmetric, la, ra, ls, rs. The default is left−symmetric.
When setting the failure mode for level faulty, the options are: write−transient, wt, read−transient, rt, write−persistent, wp, read−persistent, rp, write−all, read−fixable, rf, clear, flush, none.
Each failure mode can be followed by a number, which is used as a period between fault generation. Without a number, the fault is generated once on the first relevant request. With a number, the fault will be generated after that many requests, and will continue to be generated every time the period elapses.
Multiple failure modes can be current simultaneously by using the −−grow option to set subsequent failure modes.
"clear" or "none" will remove any pending or periodic failure modes, and "flush" will clear any persistent faults.
To set the parity with −−grow, the level of the array ("faulty") must be specified before the fault mode is specified.
Finally, the layout options for RAID10 are one of ’n’, ’o’ or ’f’ followed by a small number. The default is ’n2’. The supported options are:
’n’ signals ’near’ copies. Multiple copies of one data block are at similar offsets in different devices.
’o’ signals ’offset’ copies. Rather than the chunks being duplicated within a stripe, whole stripes are duplicated but are rotated by one device so duplicate blocks are on different devices. Thus subsequent copies of a block are in the next drive, and are one chunk further down.
’f’ signals ’far’ copies (multiple copies have very different offsets). See md(4) for more detail about ’near’ and ’far’.
The number is the number of copies of each datablock. 2 is normal, 3 can be useful. This number can be at most equal to the number of devices in the array. It does not need to divide evenly into that number (e.g. it is perfectly legal to have an ’n2’ layout for an array with an odd number of devices).
−−parity=
same as −−layout (thus explaining the p of −p).
−b, −−bitmap=
Specify a file to store a write-intent bitmap in. The file should not exist unless −−force is also given. The same file should be provided when assembling the array. The file may not reside on a filesystem that is built on top of the array the bitmap file is for or else a kernel deadlock will occur. This is not a bug, it’s a feature. If the word internal is given, then the bitmap is stored with the metadata on the array, and so is replicated on all devices. If the word none is given with −−grow mode, then any bitmap that is present is removed.
To help catch typing errors, the filename must contain at least one slash (’/’) if it is a real file (not ’internal’ or ’none’).
Note: external bitmaps are only known to work on ext2 and ext3. Storing bitmap files on other filesystems may result in serious problems.
Note: The choice of internal versus external bitmap can have a drastic impact on performance. While an internal bitmap is the most convenient as it doesn’t require a device totally separate from the array on which to store the bitmap file, it has a larger impact on performance than an external bitmap. This is because we can’t predict which device in the array might fail, so we store a copy of the bitmap on every device in the array when using an internal bitmap. This means that prior to allowing a write to a section of the array that is currently marked clean in the bitmap, we must issue a write to change the bit for that section of the array from clean to dirty, and must wait for the bitmap write to complete on all of the array devices before the pending write to the array data area can proceed. Especially if the array is under heavy load, these syncronous writes can drastically impact performance. An external bitmap file is less convenient, but there is only one copy of the bitmap, so there is only one bitmap write that must complete before the pending write to the array data can proceed. In addition, if your bitmap file device is not heavily loaded, and the array is, then you will notice a considerable performance benefit from the fact that bitmap writes are not competing with array reads/writes. The performance impact of this option can be somewhat mitigated by appropriate selection of a bitmap chunk size (next option).
−−bitmap−chunk=
Set the chunksize of the bitmap. Each bit corresponds to that many Kilobytes of storage. When using a file based bitmap, the default is to use the smallest size that is at-least 4 and requires no more than 2^21 chunks. When using an internal bitmap, the chunksize is automatically determined to make best use of available space.
Note: This option can drastically effect performance of the array. The more granular the bitmap is, then the more frequently writes will trigger syncronous bitmap updates and be delayed until the bitmap update is complete. The trade off is that a more granular bitmap means a shorter array resync time after any event causes the array to go down unclean. Given raw drive speeds can be in excess of 100MB/s on modern SATA/SAS drives, any bitmap chunk up to 262144 (256MB) can generally be synced in a matter of just a few seconds. Smaller chunks can be synced faster, but you reach a point of diminishing returns that is quickly offset by the increased write performance degradation seen in every day operation. Considering that the smaller bitmap chunk sizes will only ever be a benefit on rare occasions (hopefully never), but that you will pay for a small bitmap chunk every single day, it is recommended that you select the largest bitmap chunk size you feel comforable with.
−W, −−write−mostly
subsequent devices lists in a −−build, −−create, or −−add command will be flagged as ’write-mostly’. This is valid for RAID1 only and means that the ’md’ driver will avoid reading from these devices if at all possible. This can be useful if mirroring over a slow link.
−−write−behind=
Specify that write-behind mode should be enabled (valid for RAID1 only). If an argument is specified, it will set the maximum number of outstanding writes allowed. The default value is 256. A write-intent bitmap is required in order to use write-behind mode, and write-behind is only attempted on drives marked as write-mostly.
−−assume−clean
Tell mdadm that the array pre-existed and is known to be clean. It can be useful when trying to recover from a major failure as you can be sure that no data will be affected unless you actually write to the array. It can also be used when creating a RAID1 or RAID10 if you want to avoid the initial resync, however this practice — while normally safe — is not recommended. Use this only if you really know what you are doing.
−−backup−file=
This is needed when −−grow is used to increase the number of raid-devices in a RAID5 if there are no spare devices available. See the section below on RAID_DEVICE CHANGES. The file should be stored on a separate device, not on the raid array being reshaped.
−N, −−name=
Set a name for the array. This is currently only effective when creating an array with a version-1 superblock. The name is a simple textual string that can be used to identify array components when assembling.
−R, −−run
Insist that mdadm run the array, even if some of the components appear to be active in another array or filesystem. Normally mdadm will ask for confirmation before including such components in an array. This option causes that question to be suppressed.
−f, −−force
Insist that mdadm accept the geometry and layout specified without question. Normally mdadm will not allow creation of an array with only one device, and will try to create a raid5 array with one missing drive (as this makes the initial resync work faster). With −−force, mdadm will not try to be so clever.
−a, −−auto{=no,yes,md,mdp,part,p}{NN}
Instruct mdadm to create the device file if needed, possibly allocating an unused minor number. "md" causes a non-partitionable array to be used. "mdp", "part" or "p" causes a partitionable array (2.6 and later) to be used. "yes" requires the named md device to have a ’standard’ format, and the type and minor number will be determined from this. See DEVICE NAMES below.
The argument can also come immediately after "−a". e.g. "−ap".
If −−auto is not given on the command line or in the config file, then the default will be −−auto=yes.
If −−scan is also given, then any auto= entries in the config file will override the −−auto instruction given on the command line.
For partitionable arrays, mdadm will create the device file for the whole array and for the first 4 partitions. A different number of partitions can be specified at the end of this option (e.g. −−auto=p7). If the device name ends with a digit, the partition names add a ’p’, and a number, e.g. "/dev/home1p3". If there is no trailing digit, then the partition names just have a number added, e.g. "/dev/scratch3".
If the md device name is in a ’standard’ format as described in DEVICE NAMES, then it will be created, if necessary, with the appropriate number based on that name. If the device name is not in one of these formats, then a unused minor number will be allocated. The minor number will be considered unused if there is no active array for that number, and there is no entry in /dev for that number and with a non-standard name.
−−symlink=no
Normally when −−auto causes mdadm to create devices in /dev/md/ it will also create symlinks from /dev/ with names starting with md or md_. Use −−symlink=no to suppress this, or −−symlink=yes to enforce this even if it is suppressing mdadm.conf.
−u, −−uuid=
uuid of array to assemble. Devices which don’t have this uuid are excluded
−m, −−super−minor=
Minor number of device that array was created for. Devices which don’t have this minor number are excluded. If you create an array as /dev/md1, then all superblocks will contain the minor number 1, even if the array is later assembled as /dev/md2.
Giving the literal word "dev" for −−super−minor will cause mdadm to use the minor number of the md device that is being assembled. e.g. when assembling /dev/md0, −−super−minor=dev will look for super blocks with a minor number of 0.
−N, −−name=
Specify the name of the array to assemble. This must be the name that was specified when creating the array. It must either match the name stored in the superblock exactly, or it must match with the current homehost prefixed to the start of the given name.
−f, −−force
Assemble the array even if some superblocks appear out-of-date
−R, −−run
Attempt to start the array even if fewer drives were given than were present last time the array was active. Normally if not all the expected drives are found and −−scan is not used, then the array will be assembled but not started. With −−run an attempt will be made to start it anyway.
−−no−degraded
This is the reverse of −−run in that it inhibits the startup of array unless all expected drives are present. This is only needed with −−scan, and can be used if the physical connections to devices are not as reliable as you would like.
−a, −−auto{=no,yes,md,mdp,part}
See this option under Create and Build options.
−b, −−bitmap=
Specify the bitmap file that was given when the array was created. If an array has an internal bitmap, there is no need to specify this when assembling the array.
−−backup−file=
If −−backup−file was used to grow the number of raid-devices in a RAID5, and the system crashed during the critical section, then the same −−backup−file must be presented to −−assemble to allow possibly corrupted data to be restored.
−U, −−update=
Update the superblock on each device while assembling the array. The argument given to this flag can be one of sparc2.2, summaries, uuid, name, homehost, resync, byteorder, devicesize, or super−minor.
The sparc2.2 option will adjust the superblock of an array what was created on a Sparc machine running a patched 2.2 Linux kernel. This kernel got the alignment of part of the superblock wrong. You can use the −−examine −−sparc2.2 option to mdadm to see what effect this would have.
The super−minor option will update the preferred minor field on each superblock to match the minor number of the array being assembled. This can be useful if −−examine reports a different "Preferred Minor" to −−detail. In some cases this update will be performed automatically by the kernel driver. In particular the update happens automatically at the first write to an array with redundancy (RAID level 1 or greater) on a 2.6 (or later) kernel.
The uuid option will change the uuid of the array. If a UUID is given with the −−uuid option that UUID will be used as a new UUID and will NOT be used to help identify the devices in the array. If no −−uuid is given, a random UUID is chosen.
The name option will change the name of the array as stored in the superblock. This is only supported for version-1 superblocks.
The homehost option will change the homehost as recorded in the superblock. For version-0 superblocks, this is the same as updating the UUID. For version-1 superblocks, this involves updating the name.
The resync option will cause the array to be marked dirty meaning that any redundancy in the array (e.g. parity for raid5, copies for raid1) may be incorrect. This will cause the raid system to perform a "resync" pass to make sure that all redundant information is correct.
The byteorder option allows arrays to be moved between machines with different byte-order. When assembling such an array for the first time after a move, giving −−update=byteorder will cause mdadm to expect superblocks to have their byteorder reversed, and will correct that order before assembling the array. This is only valid with original (Version 0.90) superblocks.
The summaries option will correct the summaries in the superblock. That is the counts of total, working, active, failed, and spare devices.
The devicesize will rarely be of use. It applies to version 1.1 and 1.2 metadata only (where the metadata is at the start of the device) and is only useful when the component device has changed size (typically become larger). The version 1 metadata records the amount of the device that can be used to store data, so if a device in a version 1.1 or 1.2 array becomes larger, the metadata will still be visible, but the extra space will not. In this case it might be useful to assemble the array with −−update=devicesize. This will cause mdadm to determine the maximum usable amount of space on each device and update the relevant field in the metadata.
−−auto−update−homehost
This flag is only meaningful with auto-assembly (see discussion below). In that situation, if no suitable arrays are found for this homehost, mdadm will rescan for any arrays at all and will assemble them and update the homehost to match the current host.
−a, −−add
add listed devices to a live array. When the array is in a degraded state and you add a device, the device will be added as a spare device and reconstruction on to the spare device will commence. Upon completion of the reconstruction, the device will be transitioned to an active device. If you add more devices than the array’s normal capacity of active devices, then they are automatically added as hot spare devices. In order to utilize the spare devices, use the Grow mode of mdadm to increase the number of active devices in the array.
−−re−add
re-add a device that was recently removed from an array. This only applies to devices that were part of an array built without a persistent superblock, and for which a write intent bitmap exists. In this isolated case, the kernel will treat this device as a previous member of the array even though there is no superblock to tell it to do so. For all add operations involving arrays with persistent superblocks, use the −−add command above and the kernel will automatically determine whether a full resync or partial resync is needed based upon the superblock state and the write intent bitmap state (if it exists).
−r, −−remove
remove listed devices. They must not be active. i.e. they should be failed or spare devices. As well as the name of a device file (e.g. /dev/sda1) the words failed and detached can be given to −−remove. The first causes all failed device to be removed. The second causes any device which is no longer connected to the system (i.e an ’open’ returns ENXIO) to be removed. This will only succeed for devices that are spares or have already been marked as failed.
−f, −−fail
mark listed devices as faulty. As well as the name of a device file, the word detached can be given. This will cause any device that has been detached from the system to be marked as failed. It can then be removed.
−−set−faulty
same as −−fail.
−−write−mostly
Subsequent devices that are added or re-added will have the ’write-mostly’ flag set. This is only valid for RAID1 and means that the ’md’ driver will avoid reading from these devices if possible.
−−readwrite
Subsequent devices that are added or re-added will have the ’write-mostly’ flag cleared.
Each of these options require that the first device listed is the array to be acted upon, and the remainder are component devices to be added, removed, or marked as faulty. Several different operations can be specified for different devices, e.g.
mdadm /dev/md0 −−add /dev/sda1 −−fail /dev/sdb1 −−remove /dev/sdb1
Each operation applies to all devices listed until the next operation.
If an array is using a write-intent bitmap, then devices which have been removed can be re-added in a way that avoids a full reconstruction but instead just updates the blocks that have changed since the device was removed. For arrays with persistent metadata (superblocks) this is done automatically. For arrays created with −−build mdadm needs to be told that this device was removed recently by using −−re−add instead of −−add command (see above).
Devices can only be removed from an array if they are not in active use, i.e. they must be spares or failed devices. To remove an active device, it must first be marked as faulty.
−Q, −−query
Examine a device to see (1) if it is an md device and (2) if it is a component of an md array. Information about what is discovered is presented.
−D, −−detail
Print detail of one or more md devices.
−Y, −−export
When used with −−detail or −−examine, output will be formatted as key=value pairs for easy import into the environment.
−E, −−examine
Print content of md superblock on device(s).
−−sparc2.2
If an array was created on a 2.2 Linux kernel patched with RAID support, the superblock will have been created incorrectly, or at least incompatibly with 2.4 and later kernels. Using the −−sparc2.2 flag with −−examine will fix the superblock before displaying it. If this appears to do the right thing, then the array can be successfully assembled using −−assemble −−update=sparc2.2.
−X, −−examine−bitmap
Report information about a bitmap file. The argument is either an external bitmap file or an array component in case of an internal bitmap.
−R, −−run
start a partially built array.
−S, −−stop
deactivate array, releasing all resources.
−o, −−readonly
mark array as readonly.
−w, −−readwrite
mark array as readwrite.
−−zero−superblock
If the device contains a valid md superblock, the block is overwritten with zeros. With −−force the block where the superblock would be is overwritten even if it doesn’t appear to be valid.
−t, −−test
When used with −−detail, the exit status of mdadm is set to reflect the status of the device.
−W, −−wait
For each md device given, wait for any resync, recovery, or reshape activity to finish before returning. mdadm will return with success if it actually waited for every device listed, otherwise it will return failure.
−−rebuild−map, −r
Rebuild the map file (/var/run/mdadm/map) that mdadm uses to help track which arrays are currently being assembled.
−−run, −R
Run any array assembled as soon as a minimal number of devices are available, rather than waiting until all expected devices are present.
−−scan, −s
Only meaningful with −R this will scan the map file for arrays that are being incrementally assembled and will try to start any that are not already started. If any such array is listed in mdadm.conf as requiring an external bitmap, that bitmap will be attached first.
−m, −−mail
Give a mail address to send alerts to.
−p, −−program, −−alert
Give a program to be run whenever an event is detected.
−y, −−syslog
Cause all events to be reported through ’syslog’. The messages have facility of ’daemon’ and varying priorities.
−d, −−delay
Give a delay in seconds. mdadm polls the md arrays and then waits this many seconds before polling again. The default is 60 seconds.
−f, −−daemonise
Tell mdadm to run as a background daemon if it decides to monitor anything. This causes it to fork and run in the child, and to disconnect form the terminal. The process id of the child is written to stdout. This is useful with −−scan which will only continue monitoring if a mail address or alert program is found in the config file.
−i, −−pid−file
When mdadm is running in daemon mode, write the pid of the daemon process to the specified file, instead of printing it on standard output.
−1, −−oneshot
Check arrays only once. This will generate NewArray events and more significantly DegradedArray and SparesMissing events. Running
mdadm −−monitor −−scan −1
from a cron script will ensure regular notification of any degraded arrays.
−t, −−test
Generate a TestMessage alert for every array found at startup. This alert gets mailed and passed to the alert program. This can be used for testing that alert message do get through successfully.