From mboxrd@z Thu Jan  1 00:00:00 1970
From: Roman Mamedov <roman@rm.pp.ru>
Subject: Re: RAID6 and crashes (reporting back re. --bitmap)
Date: Fri, 11 Jun 2010 10:55:21 +0600
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To: Miles Fidelman <mfidelman@meetinghouse.net>
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On Fri, 11 Jun 2010 00:46:47 -0400
Miles Fidelman <mfidelman@meetinghouse.net> wrote:

> Looks like my original --bitmap internal creation set a very large chunk=
=20
> size initially
>=20
> md3 : active raid6 sda4[0] sdd4[3] sdc4[2] sdb4[1]
>        947417088 blocks level 6, 64k chunk, algorithm 2 [4/4] [UUUU]
>        bitmap: 6/226 pages [24KB], 1024KB chunk
>=20
> unless that --bitmap-chunk=3D131072 recommendation is translates to=20
> 131072KB (if so, are you really running 131MB chunks?)

Yes, this is correct.
This will only mean that after an unclean shutdown, at least 128MB-sized
areas of the array will be invalidated for a resync, and not smaller areas
with 1MB-granularity like on yours currently. 128 megabytes is just about 1
second of read throughput on modern drives, so I am okay with that. Several
128MB-windows here and there are still faster to resync than the whole arra=
y.
And this had an extemely good effect on write performance for me (increased=
 it
by more than 1.5x) compared to a small chunk. Test for yourself, first with=
out
the bitmap, then with various chunk sizes of it (ensure there's no other lo=
ad
on the array, and note the speeds):

dd if=3D/dev/zero of=3D/your-raid/zerofile bs=3D1M count=3D2048 conv=3Dnotr=
unc,fdatasync

--=20
With respect,
Roman

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