Re: [zfs-discuss] Performance of partition based SWAP vs. ZFS zvol SWAP
On Wed, 27 Jan 2010, RayLicon wrote: If no one has any data on this issue then fine, but I didn't waste my time posting to this site to get responses that simply say -don't swap Perhaps you can set up a test environment, measure this in a scientific way, and provide a formal summary for our edification? Thanks, 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
Re: [zfs-discuss] Performance of partition based SWAP vs. ZFS zvol SWAP
> "ag" == Andrew Gabriel writes: ag> is your working set size bigger than memory (thrashing), n...no, not...not exactly. :) ag> or is swapping likely to be just a once-off event or ag> infrequently repeated? once-off! or...well...repeated, every time the garbage collector runs. ag> You probably need to forget most of what you learned about ag> swapping 25 years ago, when systems routinely swapped, and ag> technology was very different. yes, some Lisp machines had integrated swapper/garbagecollectors. Now we have sbrk() + gc. dumb! We used to not worry about overcommitting because refusing to overcommit just meant some of the allocated swap space would never get written. It was a little bit foolish because the threat of thrashing means, whenever swap's involved, you're basically overcommitted, but it let us feel better. Now that we're not using swap, failure to overcommit seems rather wasteful. At the very least you should allow the ARC cache to grow into memory reserved for an allocation, then boot the ARC out of it if the process actually writes to more than you thought it would and you need to keep a commitment you thought you wouldn't. ag> solid state disk swap devices, smart! it might turn out to be good for ebooks and other power-constrained devices, too, because DRAM uses battery: swapping to conserve energy rather than RAM. It might be worth tracking pages in a more complicated way than we're now doing if the goal is to evacuate RAM and power it down, so maybe holding onto ancient swap wisdom and code isn't as helpful to this as it might seem. The point, keep swap on ZFS so you can grow/shrink/delete it as fashion changes, is good. But the OP's question still stands: does ZFS swap perform almost as well as raw device swap, or is it worth partitioning disks if you insist on actually using swap? I guess no one knows. pgpXR10g3t6iA.pgp Description: PGP signature ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] Performance of partition based SWAP vs. ZFS zvol SWAP
On Jan 27, 2010, at 12:25 PM, RayLicon wrote: > Ok ... > > Given that ... yes, we all know that swapping is bad (thanks for the > enlightenment). > > To Swap or not to Swap isn't releated to this question, and besides, even if > you don't page swap, other mechanisms can still claim swap space, such as the > tmp file system. > > The question is simple, "IF" you have to swap (for whatever reason), then > which of two alternatives is better (separate disk partitons on multiple > disks, or zvol ZFS stripes or mirrors - and why). Swap is striped across swap devices in 1 MB chunks. In other words, striping under the stripe won't make much difference, especially if you are using ZFS dynamic striping. > If no one has any data on this issue then fine, but I didn't waste my time > posting to this site to get responses that simply say -don't swap The data is that 5 orders of magnitude is the latency difference between RAM and HDDs. Double the performance of your swap devices and you are still looking at 5 orders of magnitude. -- richard ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] Performance of partition based SWAP vs. ZFS zvol SWAP
LICON, RAY (ATTPB) wrote: Thanks for the reply. In many situations, the hardware design isn't up to me and budgets tend to dictate everything these days. True, nobody wants to swap, but the question is "if" you had to -- what design serves you best. Independent swap slices or putting it all under control of zfs. It depends why you need to swap, i.e. why are you using more memory than you have, and is your working set size bigger than memory (thrashing), or is swapping likely to be just a once-off event or infrequently repeated? You probably need to forget most of what you learned about swapping 25 years ago, when systems routinely swapped, and technology was very different. Disks have got faster over that period, probably of the order 100 times faster. However, CPUs have got 100,000 times faster, so in reality a disk looks to be 1000 times slower from the CPU's standpoint than it did 25 years ago. This means that CPU cycles lost due to swapping will appear to have a proportionally much more dire effect on performance than they did many years back. There are lots more options available today than there were when systems routinely swapped. A couple of examples that spring to mind... ZFS has been explicitly designed to swap it's own cache data, only we don't call it swapping - we call it an L2ARC or ReadZilla. So if you have a system where the application is going to struggle with main memory, you might configure ZFS to significantly reduce it's memory buffer (ARC), and instead give it an L2ARC on a fast solid state disk. This might result in less performance degradation in some systems where memory is short, depending heavily on the behaviour of the application. If you do have to go with brute force old style swapping, then you might want to invest in solid state disk swap devices, which will go some way towards reducing the factor of 1000 I mentioned above. (Take note of aligning swap to the 4k flash i/o boundaries.) Probably lots of other possibilities too, given more than a couple of minutes thought. -- Andrew ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] Performance of partition based SWAP vs. ZFS zvol SWAP
Ok ... Given that ... yes, we all know that swapping is bad (thanks for the enlightenment). To Swap or not to Swap isn't releated to this question, and besides, even if you don't page swap, other mechanisms can still claim swap space, such as the tmp file system. The question is simple, "IF" you have to swap (for whatever reason), then which of two alternatives is better (separate disk partitons on multiple disks, or zvol ZFS stripes or mirrors - and why). If no one has any data on this issue then fine, but I didn't waste my time posting to this site to get responses that simply say -don't swap -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] Performance of partition based SWAP vs. ZFS zvol SWAP
Has anyone done research into the performance of SWAP on the traditional partitioned based SWAP device as compared to a SWAP area set up on ZFS with a zvol? I can find no best practices for this issue. In the old days it was considered important to separate the swap devices onto individual disks (controllers) and select the outer cylinder groups for the partition (to gain some read speed). How does this compare to creating a single SWAP zvol within a rootpool and then mirroring the rootpool across two separate disks? Ray Licon at&t, Global Systems and Applications Office: 310-762-6968 Pager: 310-681-0140 ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] Performance of partition based SWAP vs. ZFS zvol SWAP
RayLicon wrote: Has anyone done research into the performance of SWAP on the traditional partitioned based SWAP device as compared to a SWAP area set up on ZFS with a zvol? I can find no best practices for this issue. In the old days it was considered important to separate the swap devices onto individual disks (controllers) and select the outer cylinder groups for the partition (to gain some read speed). How does this compare to creating a single SWAP zvol within a rootpool and then mirroring the rootpool across two separate disks? Best practice nowadays is to design a system so it doesn't need to swap. Then it doesn't matter what the performance of the swap device is. -- Andrew Gabriel ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] Performance of partition based SWAP vs. ZFS zvol SWAP
Has anyone done research into the performance of SWAP on the traditional partitioned based SWAP device as compared to a SWAP area set up on ZFS with a zvol? I can find no best practices for this issue. In the old days it was considered important to separate the swap devices onto individual disks (controllers) and select the outer cylinder groups for the partition (to gain some read speed). How does this compare to creating a single SWAP zvol within a rootpool and then mirroring the rootpool across two separate disks? -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
