Neil Brown wrote:
On Thursday November 1, janek_listy@xxxxx wrote:
Hello,
I have raid5 /dev/md1, --chunk=128 --metadata=1.1. On it I have
created LVM volume called 'raid5', and finally a logical volume
'backup'.
Then I formatted it with command:
mkfs.ext3 -b 4096 -E stride=32 -E resize=550292480 /dev/raid5/backup
And because LVM is putting its own metadata on /dev/md1, the ext3
partition is shifted by some (unknown for me) amount of bytes from
the beginning of /dev/md1.
I was wondering, how big is the shift, and would it hurt the
performance/safety if the `ext3 stride=32` didn't align perfectly
with the physical stripes on HDD?
It is probably better to ask this question on an ext3 list as people
there might know exactly what 'stride' does.
I *think* it causes the inode tables to be offset in different
block-groups so that they are not all on the same drive. If that is
the case, then an offset causes by LVM isn't going to make any
difference at all.
Actually, I think that all of the performance evil Doug was mentioning
will apply to LVM as well. So if things are poorly aligned, they will be
poorly handled, a stripe-sized write will not go in a stripe, but will
overlap chunks and cause all the data from all chunks to be read back
for a new raid-5 calculation.
So I would expect this to make a very large performance difference, so
even if it work it would do so slowly.
--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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