[Lustre-discuss] SSD caching of MDT

[Andreas Dilger]

On 2010-08-19, at 7:27, LaoTsao 老曹 <laotsao at gmail.com> wrote:
> IMHO,  SSD has more IOPS  then disks and has larger capacity then raid/nvram
> so it seems that SSD should help in MDS, the U want SSD in dual host env to support failover?
> regards
> 
> 
> On 8/19/2010 8:29 AM, Gregory Matthews wrote:
>> Article by Jeff Layton:
>> 
>> http://www.linux-mag.com/id/7839
>> 
>> anyone have views on whether this sort of caching would be useful for
>> the MDT? My feeling is that MDT reads are probably pretty random but
>> writes might benefit...?
>> 

The MDS is doing mostly linear writes to the journal and async checkpoint of those blocks to the filesystem (so they can be reordered and merged).

That is one reason why we have seen relatively modest performance gains from SSD in benchmarks. 

That said, I think that there is still benefit to be had from SSD _if the whole MDT is on SSD_ because I suspect the random lookup/unlink performance for full/aged filesystems will be much better.  We haven't done any benchmarks to this effect, however. 

Cheers, Andreas