Re: stride / stripe alignment on LVM ?

[Goswin von Brederlow <brederlo@informatik.uni-tuebingen.de>]

Neil Brown <neilb@suse.de> writes:

> On Thursday November 1, janek_listy@wp.pl 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.
>
> NeilBrown

Afaik that is true and I never found any significant speed difference
in ext3 no matter what stripe size I select. The natural speed
fluctuations of e.g. bonnie seem to be bigger than the difference the
stripe size option makes.

But then again I test for large files so inode operations are not that
common. You probably have to test creating lots of dirs and small
files and file deletion to see any effect.

MfG
        Goswin