On Fri, 19 Nov 2010, Lukas Czerner wrote: > On Fri, 19 Nov 2010, Christoph Hellwig wrote: > > > On Fri, Nov 19, 2010 at 08:20:58AM -0800, Greg Freemyer wrote: > > > The kernel team has been coding around some Utopian SSD TRIM > > > implementation for at least 2 years with the basic assumption that > > > SSDs can handle thousands of trims per second. Just mix em in with > > > the rest of the i/o. No problem. Intel swore to us its the right > > > thing to do. > > > > Thanks Greg, good that you told us what we've been doing. I would have > > forgot myself if you didn't remember me. > > > > > I'm still waiting to see the first benchmark report from anywhere > > > (SSD, Thin Provisioned SCSI) that the online approach used by mount -o > > > discard is a win performance wise. Linux has a history of designing > > > for reality, but for some reason when it comes to SSDs reality seems > > > not to be a big concern. > > > > Both Lukas and I have done extensive benchmarks on various SSDs and > > thinkly provisioned raids. Unfortunately most of the hardware is only > > available under NDA so we can't publish it. > > > > For the XFS side which I've looked it I can summarize that we do have > > arrays that do the online discard without measureable performance > > penalty on various workloads, and we have devices (both SSDs and arrays) > > where the overhead is incredibly huge. I can also say that doing the > > walk of the freespace btrees similar to the offline discard, but every > > 30 seconds or at a similarly high interval is a sure way to completely > > kill performance. > > > > Or in short we haven't fund the holy grail yet. > > > > Indeed we have not. But speaking of benchmarks I have just finished > quick run (well, not so quick:)) of my discard-kit for btrfs filesystem > and here are results. Note that tool used for this benchmark is > postmark, hence it might not be the realest use-case, but it provides > nice comparison between ext4 (below) and btrfs online discard > implementation (FITRIM is NOT involved). > > > (Sadly the table is too wide so you have to...well, you guys can manage > it somehow, right?). > -snip- In the sake of completeness here is an information how it was tested. 1. mkfs 2. mount -o (nodiscard|discard) 3. ./postmark 4. umount 5. repeat 1. - 4. ten times for each mnt option -snip- > > (Buffering means that C library function like fopen, fread, fwrite are > used instead of open, read, write. I have used the word buffering in the > same way as it is used in the postmark test) > > So, you can see that Btrfs handles online discard quite better than ext4 > (cca 20% difference), but it is still pretty massive performance loss on > not-so-good-but-I-have-seen-worse SSD. So, I would say that you guys > (Josef?) should at least consider the possibility of using FITRIM as well. > > Thanks! > > -Lukas > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
