[DRBD-user] external metadata on ssd vs bcache

Lionel Sausin ls at numerigraphe.com
Wed Feb 27 18:32:07 CET 2013

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

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.

Lionel Sausin.

Le 26/02/2013 23:30, James Harper a écrit :
> Smallish SSD's are dirt cheap these days and I have a spare slot in my servers so I was thinking of putting one in and running bcache (layering would look like drbd->raid0->bcache->sd[ab]). It's a bit of mucking around to set up as bcache isn't in the kernel at this time and is targeted against a kernel version that isn't what Debian uses, so it seems that maybe for now just putting the metadata on the SSD would be a much simpler option.
> I'm currently using internal metadata, does anyone have any comments on which would be the best way to go?
> Also, is there a procedure for moving metadata from internal to external?
> Thanks
> James
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