> > Use the BBWC to maintain high IOPS when X25-E's > write cache is disabled? > > It should certainly help. Note that in this case > your relatively > small battery-backed memory is accepting writes for > both the X25-E and > for the disk storage so the BBWC memory becomes 1/2 > as useful and you > are wasting some of the RAID card write performance. > > Some people here advocate putting as much > battery-backed memory on the > RAID card as possible (and with multiple RAID cards > if possible) > rather than using a slower slog SSD. Battery backed > RAM is faster > than FLASH SSDs. The only FLASH SSDs which can keep > up include their > own battery-backed (or capacitor backed) RAM. > > Regardless, if you can decouple your slog I/O path > from the main I/O > path, you should see less latency, and more > performance. This suggests > that you should use a different controller for your > X25-E's if you > can.
OK, I will disable the X25-E's write cache. But I can't prepare the different controller because there is no budget. > > At some report I have seen, write cache is > necessary for > > wear-leveling. Should I switch off the X25-E's > write cache? > > I don't know the answer to that. Intel does not seem > to provide much > detail. If you want your slog to protect as much > data as possible > when the system loses power, then it seems that you > should disable the > X25-E write cache since it is not protected. Expect > a 5X reduction in > write IOPS performance (e.g. 5000 --> 1000). I think the data is more important than the performance, so I will disable the X25-E's write cache. > > The serser has RAID card, so I can use > hardware(Adaptec's) RAID(the > > file system is ZFS). Should I use ZFS for the RAID? > > Unless the Adaptec firmware is broken so that you > can't usefully > export the disks as "JBOD" devices, then I would use > ZFS for the RAID. OK, I will use ZFS for the RAID(include boot disk). > > 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). > > A lot depends on how your mail software operates. > Some mail systems > reate 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. > > 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. I'm using qmail for the mail server on linux now, and I will replace it to solaris. I think the qmail invokes fsync whenever the server receives mail messages. And the mail server is used to relay mail received from application servers. I think slog device is useful. -- This message posted from opensolaris.org _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
