[linux-lvm] Change stripe cache for lvm raid5

[Sebastian Bachmann]

Hi again!

Actually setting the stripe cache using dmsetup reload works. You just
have to resume the device after it was reloaded. I did not read the
documentation well enough to recognize that:

$ dmsetup table
[...]
myvg-lv: 0 11525947392 raid raid5_ls 3 128 region_size 8192 3 254:5
254:6 254:7 254:8 254:9 254:10
[...]
$ dmsetup reload myvg-lv --table '0 11525947392 raid raid5_ls 5 128
stripe_cache 16384 region_size 8192 3 254:5 254:6 254:7 254:8 254:9 254:10'
$ dmsetup resume myvg-lv
$ dmsetup table
[...]
myvg-lv: 0 11525947392 raid raid5_ls 5 128 region_size 8192 stripe_cache
16534 3 254:5 254:6 254:7 254:8 254:9 254:10
[...]

If you do this, there is actually a speedup, as suggested in various
threads about mdraid.

Is there now any way to make this persistent? I saw there are udev hooks
for lvm, but I think you can not simply put the new dmsetup table
argument there...

regards
sebastian

On 01/01/2017 05:45 PM, Sebastian Bachmann wrote:
> 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
>>
>>
>>
>> _______________________________________________
>> linux-lvm mailing list
>> linux-lvm at redhat.com
>> https://www.redhat.com/mailman/listinfo/linux-lvm
>> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>>
> 
> 
> 
> _______________________________________________
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> linux-lvm at redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> 

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