вторник, 28 июня 2016 г., 17:37:57 UTC+3 пользователь Chris Laprise написал: > On 06/27/2016 09:47 PM, indoler...@gmail.com wrote: > > What are the tradeoffs between btfs, lvm, and lvm with thin provisioning > > WRT speed and space efficiency? > > > > Both btrfs and (thin) lvm do similar things with copy-on-write, though I > have not seen direct comparisons of speed between them. Btrfs is more > flexible by far, though, and its what I use for Qubes. Regular lvm is > just a hassle and IMO only good for snapshots that are immediately > created and destroyed for backup procedures. > > Qubes will automatically use reflinks whenever it clones vm disk images, > which is a COW copy that happens instantly and only allocates extra data > blocks when blocks are changed in either copy. It allows a lot of > experimentation to be done at virtually no cost in disk space, for > instance. This happens on a per-vm (actually, per-file) basis, without > having to do whole-filesystem snapshots as in lvm. This makes btrfs > potentially much more space efficient than the rest if you make use of > cloning. Btrfs also has compression. > > Its worth noting that vm images suffer from logical fragmentation > because writing to disk image files behaves like random database > updates. Because btrfs does COW on *every* write, it does make it slower > than ext4 but on Qubes not noticeably so. Qubes vm images are sparse, so > whenever deletions occur this fragments ext4 filesystems and slows them > down also. > > The fastest filesystem to use in dom0 is probably ext4 *without* lvm. > Turning off 'discard' in the vm's /rw volume may prevent some > sparse-related slowdown (at a cost in disk space). With lvm, ext4 will > probably become slower than btrfs as soon as you start making snapshots > and updates. > > Beyond that, I think its possible to assign a raw block device to a vm, > though I haven't explored that yet. Ext4 on a raw block device would be > the fastest, but not flexible or space efficient. > > Chris
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