----- Original Message ----- From: "Peter Grandi" <p...@btrfs.list.sabi.co.uk> To: "Linux fs Btrfs" <linux-btrfs@vger.kernel.org> Sent: Tuesday, 1 August, 2017 3:14:07 PM Subject: Re: Btrfs + compression = slow performance and high cpu usage
> Peter, I don't think the filefrag is showing the correct > fragmentation status of the file when the compression is used. <SNIP> As I wrote, "their size is just limited by the compression code" which results in "128KiB writes". On a "fresh empty Btrfs volume" the compressed extents limited to 128KiB also happen to be pretty physically contiguous, but on a more fragmented free space list they can be more scattered. KOS: Ok, thanks for pointing it out. I have compared the filefrag -v on another btrfs that is not fragmented and see the difference with what is happening on the sluggish one. 5824: 186368.. 186399: 2430093383..2430093414: 32: 2430093414: encoded 5825: 186400.. 186431: 2430093384..2430093415: 32: 2430093415: encoded 5826: 186432.. 186463: 2430093385..2430093416: 32: 2430093416: encoded 5827: 186464.. 186495: 2430093386..2430093417: 32: 2430093417: encoded 5828: 186496.. 186527: 2430093387..2430093418: 32: 2430093418: encoded 5829: 186528.. 186559: 2430093388..2430093419: 32: 2430093419: encoded 5830: 186560.. 186591: 2430093389..2430093420: 32: 2430093420: encoded As I already wrote the main issue here seems to be that we are talking about a "RAID5 with 128KiB writes and a 768KiB stripe size". On MD RAID5 the slowdown because of RMW seems only to be around 30-40%, but it looks like that several back-to-back 128KiB writes get merged by the Linux IO subsystem (not sure whether that's thoroughly legal), and perhaps they get merged by the 3ware firmware only if it has a persistent cache, and maybe your 3ware does not have one, but you have kept your counsel as to that. KOS: No I don't have persistent cache. Only the 512 Mb cache on board of a controller, that is BBU. If I had additional SSD caching on the controller I would have mentioned it. I was also under impression, that in a situation where mostly extra large files will be stored on the massive, the bigger strip size would indeed increase the speed, thus I went with with the 256 Kb strip size. Would I be correct in assuming that the RAID strip size of 128 Kb will be a better choice if one plans to use the BTRFS with compression? thanks, kos <SNIP> -- 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 -- 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
