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Bug#616723: marked as done (Add support for bitmap option of md device)

Your message dated Sun, 2 Mar 2014 01:54:46 +0100
with message-id <20140302005446.GA32523@mraw.org>
and subject line Re: Bug#462617: debian-installer: need RAID bitmap support (mdadm -b internal)
has caused the Debian Bug report #462617,
regarding Add support for bitmap option of md device
to be marked as done.

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-- 
462617: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=462617
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Package: debian-installer
Severity: whishlist
Tags: d-i

Latest debian installer on Squeeze Iso disk debian-6.0.0-amd64-BD-1.iso used to
install new computer.

At the level of creating md devices it is not possible to add an bitmap as I can
do with --bitmap=internal. Please add (for me internal is good enough ;-).

-- System Information:
Debian Release: 6.0
Architecture: amd64 (x86_64)

Kernel: Linux 2.6.32-5-amd64 (SMP w/8 CPU cores)

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Russell Coker <russell@coker.com.au> (2008-01-26):
> A recent (some time in the last 5 years) feature of Linux software
> RAID is a "write intent bitmap".  The purpose of this is that before
> writing to a section of disk the bitmap is altered to mark it as
> dirty.  Then if the machine experiences a power failure or other
> catastrophic event then when rebooted it will know that the section
> referenced by that bit is dirty.
> 
> Thus on reboot after a serious failure small amounts of data will be
> synchronised instead of large amounts.
> 
> This feature is not currently used by the Debian installer.  So if you
> install to a system with multiple disks in a RAID-1 array (and
> probably RAID-5 and RAID-6) then every time there is a power failure
> the disks in the RAID will have to be completely read to ensure that
> they match.
> 
> If the mdadm command that was used for the Debian installer had "-b
> internal" appended then the bitmap feature would be used and recovery
> from some failure conditions would be much faster.  There does not
> seem to be any down-side to using this.
> 
> I have not tested this with RAID-5 and RAID-6, but have tested the
> benefits of this option for RAID-1 and found it to be very useful.

The subject reminded me of some digging I've done lately to answer
someone on #debian-boot. Pasting the IRC log, anonymizing the user
nickname just in case:

2014-02-14 22:11:41[ ....] do the d-i daily's automatically add write intent bitmaps to raid arrays created in the installer?
2014-02-14 22:12:11[ KiBi] I don't know what those are.
2014-02-14 22:12:18[ ....] I think they must, a system I just installed has them. if anyone knows more details I would love to discuss
2014-02-14 22:12:55[ ....] hmm, damn my notes on them aren't public
2014-02-14 22:13:17[ ....] they are a small bitmap file that corresponds to chunks of the raid array
2014-02-14 22:13:33[ ....] before the kernel goes to write to a chunk it sets the bit
2014-02-14 22:13:54[ KiBi] so maybe partman-auto-raid is the package you want to look at?
2014-02-14 22:14:12[ ....] so if a system loses power, when it comes back up it only needs to resync the chunks that were actively being written to
2014-02-14 22:14:15[ ....] really nice
2014-02-14 22:14:36[ KiBi] unless the code for it is elsewhere, maybe on the kernel side?
2014-02-14 22:14:59[ ....] you can store them on other disks too in order to prevent the heads of a HDD needing to seek a lot
2014-02-14 22:15:07[ KiBi] ah, mdadm apparently: https://raid.wiki.kernel.org/index.php/Write-intent_bitmap
2014-02-14 22:15:15[ ....] anyway it's super cool that d-i is doing it now
2014-02-14 22:15:44[ ....] it only did some of them, the larger ones, which is also exactly what you probably want
2014-02-14 22:16:29[ ....] when you set them up you get to pick the chunk size, it's a trade off of recovery time vs how often it needs to update the bitmap
2014-02-14 22:16:50[ ....] so I am curious as to the decisions made by whoever added it
2014-02-14 22:16:53[ KiBi] autoraid-cfg does that:
2014-02-14 22:16:54[ KiBi]                 mdadm --create /dev/md$MD_NUM --auto=yes --force -R -l raid$RAID_TYPE \
2014-02-14 22:16:54[ KiBi]                       -n $DEV_COUNT $MDADM_PARAMS
2014-02-14 22:17:07[ KiBi] (in the partman-auto-raid package)
2014-02-14 22:17:20[ KiBi] might just be some changed default on the mdadm side?
2014-02-14 22:17:28[ ....] hmm, could be
2014-02-14 22:17:29[ KiBi] mjt would probably know
2014-02-14 22:17:43[ ....] since that command line doesn't list it
2014-02-14 22:17:54[ ....] well unless it's in $MDADM_PARAMS
2014-02-14 22:18:12[ KiBi] auto-raidcfg:           MDADM_PARAMS="-x $SPARE_COUNT $RAID_DEVICES $MISSING_DEVICES $SPARE_DEVICES $MISSING_SPARES"
2014-02-14 22:18:12[ KiBi] auto-raidcfg:           MDADM_PARAMS="$RAID_DEVICES"
2014-02-14 22:18:21[ KiBi] doesn't look like it
2014-02-14 22:18:25[ ....] yeah
2014-02-14 22:21:01[ KiBi] not obvious from the mdadm 3.x announces, cloning the repo..
2014-02-14 22:21:22[ KiBi] Author: NeilBrown <neilb@suse.de>
2014-02-14 22:21:22[ KiBi] Date:   Tue Mar 5 10:36:21 2013 +1100
2014-02-14 22:21:22[ KiBi]     Create: default to bitmap=internal for large arrays.
2014-02-14 22:21:36[ KiBi] mdadm-3.2.5-210-g748952f
2014-02-14 22:21:48[ KiBi] sorry, mdadm-3.3-rc1~102
2014-02-14 22:22:05[ KiBi] so the 3.3 release has it, and we got that for free when building d-i against that version?
2014-02-14 22:22:22[ KiBi] [2013-10-25] mdadm 3.3-2 MIGRATED to testing (Britney)
2014-02-14 22:22:57[ KiBi] so since debian-installer 20131211

Closing this bug report accordingly.

Mraw,
KiBi.

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