[DRBD-user] external metadata on ssd vs bcache

Adam Goryachev mailinglists at websitemanagers.com.au
Wed Feb 27 23:41:59 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.

Lionel Sausin <ls at numerigraphe.com> wrote:

>I wouldn't expect anything like the gains of
>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

Just a word of warning for anyone who is considering to use SSD's, I was recently bitten by a severe performance degradation by Linux 2.6.26 (specifically the version in debian stable), where SSD performance was reduced to about half that achievable by a single hard drive (yes seriously). There is a simple patch to resolve this, or in my case, I upgraded to 3.2 from backports.

Hope this helps someone else quicker than it took me to work it out (using 5 x 480G SSD's in RAID5 and wondering why performance sucked so badly)


Adam Goryachev
Website Managers

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