On Oct 10, 2009, at 4:11 PM, Bob Friesenhahn wrote:
On Sat, 10 Oct 2009, tak ar wrote:
I think the IOPS is important for mail server, so ZIL is useful.
The server has 48GB RAM and two(ZFS or hardware mirror) X25-E(32GB)
for ZIL(slog). I understand the ZIL needs half of RAM.
There is a difference between synchronous IOPS and async "IOPS"
since synchronous writes require that data be written right away
while async I/O can be written later. Postponed writes are much
more efficient.
If the mail software invokes fsync(2) to flush a mail file to disk,
then a synchronous write is required. However, there is still a
difference between opening a file with the O_DSYNC option (all
writes are synchronous) and using the fsync(2) call when the file
write is complete (only pending unwritten data is synchronous).
I'm not aware of email services using sync regularly. In my experience
with large
email services, the response time of the disks used for database and
indexes is
the critical factor (for > 600 messages/sec delivered, caches don't
matter :-)
Performance of the disks for the mail messages themselves is not as
critical.
A lot depends on how your mail software operates. Some mail systems
create a file for each mail message while others concatenate all of
the messages for one user into one file.
You may want to defer installing your X25-Es and evaluate
performance of the mail system with a DTrace tool called 'zilstat',
which is written by Richard Elling. This tool will tell you how
much and what type of synchronous write traffic you have.
Yes. In my experience, for the hardware I was using, a separate log
was better,
but not required to meet the performance goals.
-- richard
It is currently difficult to remove slog devices so it is safer to
add them if you determine they will help rather than reduce
performance.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
