ZFS: How to enable cache and logs.

[Bob Friesenhahn]

On Thu, 12 May 2011, Rick Macklem wrote:
>> The large write feature of the ZIL is a reason why we should
>> appreciate modern NFS's large-write capability and avoid anchient NFS.
>>
> The size of a write for the new FreeBSD NFS server is limited to
> MAX_BSIZE. It is currently 64K, but I would like to see it much larger.
> I am going to try increasing MAX_BSIZE soon, to see what happens.

Zfs would certainly appreciate 128K since that is its default block 
size.  When existing file content is overwritten, writing in properly 
aligned 128K blocks is much faster due to ZFS's COW algorithm and not 
needing to read the existing block.  With a partial "overwrite", if 
the existing block is not already cached in the ARC, then it would 
need to be read from underlying store before the replacement block can 
be written.  This effect becomes readily apparent in benchmarks.  In 
my own benchmarking I have found that 128K is sufficient and using 
larger multiples of 128K does not obtain much more performance.

When creating a file from scratch, zfs performs well for async writes 
if a process writes data smaller than 128K.  That might not be the 
case for sync writes.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/