On 01/04/2012 14:28, John Robinson wrote:
On 01/04/2012 11:33, David Brown wrote:
[...]
For an application like this, it would probably make sense to put the
xfs log (and the mdraid bitmap file, if you are using one) on a separate
disk - perhaps a small SSD.
Now that you've suggested that, it occurs to me that for an application
like this, the OP might be better doing his multiple slow streams to a
spool folder on SSD, and copying over to the big array of spinning rust
when each stream completes. 200 streams of 500MB is 100GB of data, so a
pair of slightly larger SSDs in RAID1 (or RAID10, to balance the reads
coming off) would do nicely as a spool area.
If the OP wants to do that, he must be careful when looking at the rated
write speeds for the SSDs. SSDs need a bit of a break on occasion to
get time to do their garbage collection - their long-term streamed write
speed is therefore lower than their short-term write speeds (and that's
the figure that will be given in the spec). But if you take that into
account, and short-partition the SSD's a little (i.e., leave 10%
unpartitioned, and put the raid1 on the partition rather than the disk)
so that there are always plenty of free blocks when they are needed,
that could be a very good solution.
This might also be a good application for bcache, FlashCache or whatever.
Cheers,
John.
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