On Wed, 27 Feb 2013 18:32:07 +0100 Lionel Sausin <[email protected]> wrote: > I wouldn't expect anything like the gains of > bcache/flashcache/enhencio. Normally internal metadata are just as > fast, thanks to the write cache of your disks and RAID adapter. Those > are much faster than SSDs and metadata are small enough. > However you may benefit from external metadata when your those caches > are saturated by writes (high throughput for a long time). > If you do have an SSD and expect big writes, give it a try and please > tell us if it really makes a difference.
My experience with an ssd for (external) meta-data says that imrovement is quite a lot! You won't get faster continous writes, that is still limited by the hdd. But you get much faster random-writes and the reason is this: - With internal meta-data on hdd, each write (or until each barrier) is followed by a disk-seek to the end of the disk where the meta-data lives followed by a seek back to where you are writing. And then you mix random writes at random positions... - With external meta-data on another hdd, your data-disk doesn't have to seek to the end of the disk anymore, step one of improvement. - With external meta-data on ssd, you are only left with the seeks during your normal random writes. With todays disks and normal usage (unless you are netflix or google), the real speed-improvement your users see/feel is not faster throughput but lower latency. Of course, using internal meta-data with the whole partition on ssd gives you the best performance, but not everyone can buy enough ssds to create a mirrored 6TB array of ssd. 3x2TB hdd + 160G ssd (for meta-data and the fast-loving databases) times two on the other hand is actually affordable... As to the original authors question: There is a manpage about drbdmeta which describes the options to dump and restore the meta-data of an offline drbd. So the action will be: - stop the drbd-volume - dump the meta-data - change the config to point the meta-data to the new place - restore the meta-data - restart the drbd-volume - wait for sync (only incremental, not a full sync) and repeat with the other node I did that with several volumes when our ssds arrived. Test the steps with a scrap-drbd-volume before doing the procedure on production-data to be sure. Have fun, Arnold
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