random comments for now
Bitmaps optimise rebuild time after a crash, or after removing and re-adding a device. They do not improve normal read/write performance, and cause a small degradation in performance.
When an array has a bitmap, a device can be removed and re-added and only blocks changes since the removal (as recorded in the bitmap) will be resynced.
Note that bitmap support is only available for RAID geometries raid0, because it is meaningless: As raid0 have no redundancy, it cannot be inconsistent. So there is nothing to record.
mdadm --grow --bitmap=internal /dev/mdX
This operation is reversible:
mdadm --grow --bitmap=none /dev/mdX
Bitmaps can also be created externally on an ext3 filesystem (which must not be on the raid device).
How Bitmaps Work by Q&A
- Why, when you first create a raid1 (mirrored) array from two drives, does mdadm insist on mirroring the contents of the first drive to the second even though the drives are entirely blank (e.g. new drives don't have anything on them).
- Well... they do have something one them - lots of zeros and ones, or maybe just zeros, or maybe just ones. Sure, you may not be interested in that data, but it is there.
- In one configuration I have, this takes about 16 hours on a 400Gb drive. When I do 5 of them simultaneously this takes 2+ days to complete. Is there some way to tell mdadm that you want to create a mirrored set but skip this rather long initial mirroring process? I don't really see that it actually accomplishes anything.
- No, there is no way to tell mdadm to skip the initial copying process. It is not clear to me that you really want to do this(*) (though on the "enough rope" principle the "--assume-clean" works in --create mode).
- I suggest you simply ignore the fact that it is doing the copy. Just keep using the array as though it wasn't. If this seems to be impacting over-all system performance, tune /proc/sys/dev/raid/speed_* (those are global parameters, check /sys/devices/virtual/block/md*/md/ for parameters established per md device) the to slow it down even more. If you reboot, it should remember where it was up to and restart from the same place (providing you are using a 2.6 kernel).
- If you have 5 of these 400Gb raid1's, then I suspect you really want to avoid the slow resync that happens after a crash. You should look into adding a bitmap write-intent log. This requires 2.6.14, and mdadm 2.1, and is as easy as:
mdadm --grow --bitmap=internal /dev/md3
- while the array is running.
- This should dramatically reduce resync time, at a possible small cost in write throughput. Some limited measurements I have done suggest up to 10% slowdown, though usually less. Possibly some tuning can make it much better.
(*) A raid array can suffer from sleeping bad blocks. i.e. blocks that you cannot read, but normally you never do (because they haven't been allocated to a file yet). When a drive fails, and you are recovering the data onto a spare, hitting that sleeper can kill your array. For this reason it is good to regularly (daily, or weekly, maybe monthly) read through the entire array making sure everything is OK. In 2.6.16 (with complete functionality in 2.6.17) you will be able to trigger a background read-test of the whole array:
echo check > /sys/block/mdX/md/sync_action
If you were to create an array with --assume-clean, then whenever you run this it will report lots of errors, though you can fix them with
echo repair > /sys/block/mdX/md/sync_action
If you are going to be doing that (and I would recommend it) then you may as well allow the initial sync, especially as you can quite happily ignore the fact that it is happening.
Bitmaps and /proc/mdstat
The /proc/mdstat page describes how to interpret the bitmap line.
Non bitmap-optimised resyncs
It should be possible to do a similar thing to arrays without bitmaps. i.e. if a device is removed and re-added and *no* changes have been made in the interim, then the add should not require a resync.
A patch allows that option. This means that when assembling an array one device at a time (e.g. during device discovery) the array can be enabled read-only as soon as enough devices are available, but extra devices can still be added without causing a resync.
Used disk space for bitmaps
It uses space that the alignment requirements of the metadata assure us is otherwise unused. For v0.90, that is limited to 60K. For 1.x it is 3K. As this is unused disk space, bitmaps can be added to an existing md device without the risk to take away space from an existing filesystem on that device.