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@redhat.com >> https://www.redhat.com/mailman/listinfo/linux-lvm >> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/ >> > > > > _______________________________________________ > linux-lvm mailing list > linux-lvm@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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