[EMAIL PROTECTED] writes: > > Note also that for most applications, the size of their IO operations > > would often not match the current page size of the buffer, causing > > additional performance and scalability issues. > > Thanks for mentioning this, I forgot about it. > > Since ZFS's default block size is configured to be larger than a page, > the application would have to issue page-aligned block-sized I/Os. > Anyone adjusting the block size would presumably be responsible for > ensuring that the new size is a multiple of the page size. (If they > would want Direct I/O to work...) > > I believe UFS also has a similar requirement, but I've been wrong > before. >
I believe the UFS requirement is that the I/O be sector aligned for DIO to be attempted. And Anton did mention that one of the benefit of DIO is the ability to direct-read a subpage block. Without UFS/DIO the OS is required to read and cache the full page and the extra amount of I/O may lead to data channel saturation (I don't see latency as an issue in here, right ?). This is where I said that such a feature would translate for ZFS into the ability to read parts of a filesystem block which would only make sense if checksums are disabled. And for RAID-Z that could mean avoiding I/Os to each disks but one in a group, so that's a nice benefit. So for the performance minded customer that can't afford mirroring, is not much a fan of data integrity, that needs to do subblock reads to an uncacheable workload, then I can see a feature popping up. And this feature is independant on whether or not the data is DMA'ed straight into the user buffer. The other feature, is to avoid a bcopy by DMAing full filesystem block reads straight into user buffer (and verify checksum after). The I/O is high latency, bcopy adds a small amount. The kernel memory can be freed/reuse straight after the user read completes. This is where I ask, how much CPU is lost to the bcopy in workloads that benefit from DIO ? At this point, there are lots of projects that will lead to performance improvements. The DIO benefits seems like small change in the context of ZFS. The quickest return on investement I see for the directio hint would be to tell ZFS to not grow the ARC when servicing such requests. -r > -j > > _______________________________________________ > 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
