Hi, I am trying to figure out how I can improve write performance of a very slow and complicated RAID-arragement (slowdrive=/dev/md123) with a fast SSD/RAM-drive as a cache (fastdrive=/dev/sda4). Write access to my server normally appears occasionally in my case (few GB at a time and long idle times). So I want that every write access to the resulting block device is committed to the cache first but I do not achieve this. I read all the old mails about the tunables and the kernel docs cache.txt and cache-policy.txt but I am doing something wrong at the moment. I created a test environment for that like this: ># fastdrive="/dev/sda4" ># slowdrive="/dev/md123" ># lvcreate --name cache --size 10G BigData $fastdrive ># lvcreate --name cacheM --size 128M BigData $fastdrive ># lvcreate --name data --size 100G BigData $slowdrive ># lvconvert --cachemode writeback --chunksize 1024K \ ># --cachepool BigData/cache --poolmetadata BigData/cacheM ># lvconvert --type cache \ ># --cachepool BigData/cache BigData/data ># dmsetup message BigData-data 0 migration_threshold 0 # see below ># dmsetup message BigData-data 0 random_threshold 0 ># dmsetup message BigData-data 0 sequential_threshold 0 ># dmsetup message BigData-data 0 write_promote_adjustment 0 ># dd if=/dev/zero of=/dev/mapper/BigData-data \ ># bs=1G count=1 oflag=direct I monitored the last 'dd' with iostat and only see access to the slow device. This output confirms it: ># dmsetup status ># BigData-data: 0 209715200 cache 8 29/32768 2048 0/10240 0 230 \ ># 0 2048 0 0 0 1 writeback 2 migration_threshold 0 mq 10 \ ># random_threshold 0 sequential_threshold 0 \ ># discard_promote_adjustment 1 read_promote_adjustment 4 \ ># write_promote_adjustment 0 Essentially saying that my cache is empty. No dirty blocks. With migration_threshold set to a high number (99999999) I see write access to the cache. But dd blocks until everything is written to the slow device. It looks like a writethru. Is it possible to influence the write/cache behavior? Greetings from Germany! _______________________________________________ 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/
