On 24/03/2020 17:37, Simon Matter via CentOS wrote: > Hi, > >> Hi list, >> >> I'm building a NFS server on top of CentOS 8. >> It has 8 x 8 TB HDDs and 2 x 500GB SSDs. >> The spinning drives are in a RAID-6 array. They are 4K sector size. >> The SSDs are in RAID-1 array and with a 512bytes sector size. >> >> >> I want to use the SSDs as a cache using dm-cache. So here what I've done >> so far: >> /dev/sdb ==> SSD raid1 array >> /dev/sdd ==> spinning raid6 array > Looks like you're using a hardware raid controller, right? Yes it's a "PERC H740P Mini" with 8GB of cache. > >> I've added "allow_mixed_block_sizes = 1" to lvm.conf to be able to add >> sdb and sdd in the same VG (because of the sector size missmatch). But >> as the LVs will only use one and only one PV I guess it's OK. >> >> # lvcreate -L 500M -n lv_cache_meta VGnfs /dev/sdb >> # lvcreate -l +100%FREE -n lv_cache VGnfs /dev/sdb >> # lvconvert --type cache-pool /dev/VGnfs/lv_cache --poolmetadata >> /dev/VGnfs/lv_cache_meta >> # lvcreate -l +100%FREE -n LVnfs VGnfs /dev/sdd >> # lvconvert --type cache /dev/VGnfs/LVnfs --cachepool /dev/VGnfs/lv_cache >> # lvconvert --cachemode writeback --type cache /dev/VGnfs/LVnfs >> --cachepool /dev/VGnfs/lv_cache >> # mkfs.xfs /dev/VGnfs/LVnfs >> >> And now I'm asking myself if I'm not doing something somewhat dangerous >> for a server that will be critical when in production :] > I can't comment here as I've never used SSD as a cache the way you do it. > >> The server has 128GB of RAM so maybe the caching in memory will be >> sufficient to achieve good performance ? > The 128GB will of course help as read cache but what about write cache? If > your raid hardware has battery/flash based write cache and is big enough, > then you could set it in write back mode and make use of it. Yes it has, the current mode is write back until the battery condition is problematic. > I'm not sure > your proposed configuration is safe enough. Yes, I'm thinking about abandoning it, especially since I'm achieving acceptable performance without another layer of cache. I was just wondering if other people were using it in production. Thanks anyway, take care kfx > > Regards, > Simon > > _______________________________________________ > CentOS mailing list > CentOS at centos.org > https://lists.centos.org/mailman/listinfo/centos