[linux-lvm] Change stripe cache for lvm raid5

[Sebastian Bachmann]

Hi!

Just a sidenote: I tried to set the stripe_cache option via dmsetup
reload in a debian jessie VM. With the 3.16 kernel, it crashed with a
null pointer dereference error in raid5_set_cache_size function.
I upgraded to a newer kernel version (4.8.11) and tried it again,
the cache size can be set there using reload:

Jan  1 15:44:56 jessie kernel: [   33.913336] device-mapper: raid: 16384
stripe cache entries


I then upgraded the kernel on my machine with the RAID5 and found out,
that even without any cache settings, the RAID is now two to four times
faster in writing than before. I now get around 80-100MB/s.
It seems that there were a lot of changes between 3.16 and 4.8 regarding
speed, not only fixing this crash.

I tried now setting the stripe cache size on the RAID5 but the speed
does not get higher though...
According to the numerous reports on the internet, the impact on write
performance should be great - so it would still be interesting if the
stripe_cache is actually set correctly when using dmsetup reload (if I
run dmsetup table again, i still see the old line) and if it somehow can
be set persistently with LVM.

regards
sebastian

On 01/01/2017 03:15 PM, Sebastian Bachmann wrote:
> Hi!
> I'm using a LVM RAID5 on a machine but the write performance is pretty poor
> (about 20-30MB/s),
> where the read performance is quite good (about 280MB/s).
> I read about the stripe_cache_size for md raid and as far as I understand LVM
> raid, it uses md as well.
> In the design document for lvm2-raid
> (https://git.fedorahosted.org/cgit/lvm2.git/tree/doc/lvm2-raid.txt) I can find
> that a option --stripecache is specified there, but as it seems those options
> were never implemented.
> 
> Is it possible to set the stripe cache size somewhere else? It seems to me that
> lvm2 uses dmsetup to create the raid, where a stripe cache can be set while
> creation. But it seems to me there is no interface to change such values
> later on?
> 
> Thanks in advance!
> 
> regards
> Sebastian
> 
> 
> 
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> 

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