[zfs-discuss] Improving snapshot write performance

Ian Collins
2012-04-11T19:55:06Z

On 04/12/12 04:17 AM, Richard Elling wrote:
> On Apr 11, 2012, at 1:34 AM, Ian Collins wrote:
>
>> I use an application with a fairly large receive data buffer (256MB)
>> to replicate data between sites.
>>
>> I have noticed the buffer becoming completely full when receiving
>> snapshots for some filesystems, even over a slow (~2MB/sec) WAN
>> connection.  I assume this is due to the changes being widely
scattered.
>
> Widely scattered on the sending side, receiving side should be mostly
> contiguous...

That''s what I originally thought.

> unless you are mostly full or there is some other cause of slow
> writes. The usual disk-oriented
> performance analysis will show if this is the case. Most likely,
> something else is going on here.
>

Odd.  The pool is a single iSCSI volume exported from a 7320 and there
is 18TB free.

I see the same issues with local replications on our LAN.  The
filesystems that appear to write slowly are ones containing many small
files, such as office documents.

Over the WAN, the receive buffer high water mark is usually the TCP
receive window size, except for the apparently slow filesystems.

I''ll add some more diagnostics.

--
Ian.