[pve-devel] Speed up PVE Backup

Eneko Lacunza elacunza at binovo.es
Thu Jul 21 08:47:24 CEST 2016

El 20/07/16 a las 17:46, Dietmar Maurer escribió:
>> This is called from restore_extents, where a comment precisely says "try
>> to write whole clusters to speedup restore", so this means we're writing
>> 64KB-8Byte chunks, which is giving a hard time to Ceph-RBD because this
>> means lots of ~64KB IOPS.
>> So, I suggest the following solution to your consideration:
>> - Create a write buffer on startup (let's asume it's 4MB for example, a
>> number ceph rbd would like much more than 64KB). This could even be
>> configurable and skip the buffer altogether if buffer_size=cluster_size
>> - Wrap current "restore_write_data" with a
>> "restore_write_data_with_buffer", that does a copy to the 4MB buffer,
>> and only calls "restore_write_data" when it's full.
>>       * Create a new "flush_restore_write_data_buffer" to flush the write
>> buffer when device restore reading is complete.
>> Do you think this is a good idea? If so I will find time to implement
>> and test this to check whether restore time improves.
> We store those 64KB blocks out of order, so your suggestion will not work
> in general.
But I suppose they're mostly ordered?
> But you can try to assemble larger blocks, and write them once you get
> an out of order block...
Yes, this is the plan.
> I always thought the ceph libraries does (or should do) that anyways?
> (write combining)
Reading the docs:

It should be true when write-back rbd cache is activated. This seems to 
be the default, but maybe we're using disk cache setting on restore too?

I'll try to change the disk cache setting and will report the results.


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