Thought I'd do a quick summary of my results - very subjective really, so take with a pinch of salt.
As I suspected, raw disk becnhmarks, either from within the VM (Crystal DiskMark) or on the host (dd, bonnie++) while interesting, arent' a very good guide to actual application performance improvements. In the end I ran a number of std application usage timings - a long build process, Eclipse startup and builds, UI responsiveness. With all the different cache strategies I tried, disk bencharks gave widely varying results, while app benchmarking they where all very similar. dm-Cache: - Complicated to setup - Fiddly and error prone to manage - Needs custom init scripts - No auto ignoring of bulk reads/writes (disk cloning, backups) - I managed to destroy the underlying file system while attempting to flush and dismount the write cache. - Did give good results for reads/writes bcache - User tools have to be compiled and installed - can't be used with existing file systems - No auto ignoring of bulk reads/writes (disk cloning, backups) - Needs custom init scripts - 3.10 kernel version is hopelessly buggy. Trashed the file system - No tools for uninstalling. Required a hard result and then I had to use dd to overwrite the partition table to remove the cache store. I then blacklisted the module :) EnhanceIO - Has to be compiled and installed - *can* be used with existing file systems - Can be created/edited/destroyed on the fly - No auto ignoring of bulk reads/writes (disk cloning, backups) - persistent between reboots (udev rules) - Good results on reads/writes - Unfortunately when I combined it with an external ext4 journal I got data corruption ZFS (zfsonlinux.org) - Used the kernel module - has a repo, but required kernel headers and dkms. - Builtin support for journal and read caching using multiple SSD's - ZFS! with all the zfs goodies, raid, striping, snapshots, backups, pool managment. - Auto ignoring of bulk reads/writes (disk cloning, backups) - good tools for management and reporting disk/cache stats and errors - I restricted it to 1GB RAM on Hosts - Good results on reads/writes - No support for O_DIRECT so I had to disable glusters io cache, which is recommenced anyway for virtual stores. ZFS was the clear winner - ease of management, reliability, flexibility. Its going to make expanding the stores so much easier in the future. ZFS + Gluster + SSD's caches seems to be a winner for shared HA storage to me. -- Lindsay
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