fantastic feature! i`m curious: can btrfs support more than one compression scheme at the same time, i.e. is compression "pluggable" ? lzo compression coming to my mind, as this is giving real-time compession and may even speed up disk access. compression ratio isn`t too bad, but speed is awesome and doesn`t need as much cpu as gzip. experimental lzo compression in zfs-fuse showed that it could compress tarred kernel-source with 2.99x compressratio (where gzip gave 3.41x), so maybe lzo is a better algorithm for realtime filesystem compression... regards roland From: Chris Mason <chris.mason <at> oracle.com> Subject: Re: Compressed Filesystem Newsgroups: gmane.comp.file-systems.btrfs Date: 2008-10-29 20:08:42 GMT (6 weeks, 5 days, 1 hour and 53 minutes ago) On Wed, 2008-10-29 at 12:14 -0600, Anthony Roberts wrote: > Hi, I have a few questions about this: > > > Compression is optional and off by default (mount -o compress to enable > > it). When enabled, every file is compressed. > > Do you know what the CPU load is like with this enabled? Now that I've finally pushed the code out, you can try it ;) One part of the implementation I need to revisit is the place in the code where I do compression means that most of the time the single threaded pdflush is the one compressing. This doesn't spread the load very well across the cpus. It can be fixed, but I wanted to get the code out there. The decompression does spread across cpus, and I've gotten about 800MB/s doing decompress and checksumming on a zero filled compressed file. At the time, the disk was reading 14MB/s. > > Do you know whether data can be compressed at a sufficient rate to still > saturate the disk on recent-ish AMD/Intel CPUs? My recentish intel cpu can compress and checksum at about 120MB/s. > > If no, is the effective pre-compression I/O rate still comparable to the > disk without compression? > It depends on your disks... > I'm pretty sure that won't even matter in many cases (eg you're seeking > too much to care, or you're on a VM with lots of cores but congested > disks, or you're dealing with media files that it doesn't bother > compressing, etc), but I'm curious what sort of overhead this adds. :) > > Mostly it seems like a good tradeoff, it trades plentiful cores for scarce > disk resources. This varies quite a bit from workload to workload, in some places it'll make a big difference, but many workloads are seek bound and not bandwidth bound. -chris ____________________________________________________________________ Psssst! Schon vom neuen WEB.DE MultiMessenger gehört? Der kann`s mit allen: http://www.produkte.web.de/messenger/?did=3123 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
