On Mon, Jul 24, 2017 at 02:01:07PM -0400, Chris Mason wrote: > On 07/24/2017 10:25 AM, David Sterba wrote: > > > Thanks for the extensive historical summary, this change really deserves > > it. > > > > Decoupling the assumptions about the device's block management is really > > a good thing, mount option 'ssd' should mean that the device just has > > cheap seeks. Moving the the allocation tweaks to ssd_spread provides a > > way to keep the behaviour for anybody who wants it. > > > > I'd like to push this change to 4.13-rc3, as I don't think we need more > > time to let other users to test this. The effects of current ssd > > implementation have been debated and debugged on IRC for a long time. > > The description is great, but I'd love to see many more benchmarks. At > Facebook we use the current ssd_spread mode in production on top of > hardware raid5/6 (spinning storage) because it dramatically reduces the > read/modify/write cycles done for metadata writes.
Well, I think this is an example that ssd got misused because of the side effects of the allocation. If you observe good patterns for raid5, then the allocator should be adapted for that case, otherwise ssd/ssd_spread should be independent of the raid level. > If we're going to play around with these, we need a good way to measure > free space fragmentation as part of benchmarks, as well as the IO > patterns coming out of the allocator. Hans has a tool that visualizes the fragmentation. Most complaints I've seen were about 'ssd' itself, excessive fragmentation, early ENOSPC. Not many people use ssd_spread, 'ssd' gets turned on automatically so it has much wider impact. > At least for our uses, ssd_spread matters much more for metadata than > data (the data writes are large and metadata is small). >From the changes overview: > 1. Throw out the current ssd_spread behaviour. would it be ok for you to keep ssd_working as before? I'd really like to get this patch merged soon because "do not use ssd mode for ssd" has started to be the recommended workaround. Once this sticks, we won't need to have any ssd mode anymore ... -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
