Gentoo Archives: gentoo-user

From: Rich Freeman <rich0@g.o>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] Safeguarding strategies against SSD data loss
Date: Mon, 27 Oct 2014 17:31:17
In Reply to: Re: [gentoo-user] Safeguarding strategies against SSD data loss by Pandu Poluan
1 On Mon, Oct 27, 2014 at 12:52 PM, Pandu Poluan <pandu@××××××.info> wrote:
2 >
3 > ZoL (ZFS on Linux) nowadays is implemented using DKMS instead of FUSE, thus
4 > running in kernelspace, and (relatively) easier to put into an initramfs.
6 Sorry about that. I should have known that, but for some reason I got
7 that memory crossed in my brain... :)
9 > vdevs can grow, but they can't (yet) shrink.
11 Can you point to any docs on that, including any limitations/etc? The
12 inability to expand raid-z the way you can do so with mdadm was one of
13 the big things that has been keeping me away from zfs. I understand
14 that it isn't so important when you're dealing with large numbers of
15 disks (backblaze's storage pods come to mind), but when you have only
16 a few disks being able to manipulate them one at a time is very
17 useful. Growing is the more likely use case than shrinking. Then
18 again, at some point if you want to replace smaller drives with larger
19 ones you might want a way to remove drives from a vdev.
21 The one thing that btrfs does that is helpful here is that it works
22 with data in "chunks" and not at the whole drive level. That is,
23 block 1 on drive A is not hard-mapped to block 1 on drive B in the way
24 that it is with a traditional RAID. That makes it easy to have a
25 non-redundant set of disks and then switch it to raid1 mode while
26 leaving the existing data unmirrored - new chunks get mirrored, and
27 old ones don't, and you can run a command telling the system to copy
28 all the old data into new mirrored chunks.
30 > And putting ZFS on SSDs... not recommended. Rather, ZFS can employ
31 > SSDs to act as a 'write cache' for the spinning HDDs.
33 It can operate as a read-cache as well, right? I believe you'd need
34 separate drives/partitions for that.
36 >
37 > In my personal opinion, the 'killer' feature of ZFS is that it's built from
38 > the ground up to provide maximum data integrity.
40 That and the snapshots are actually common to both btrfs and ZFS. The
41 main advantages of ZFS over btrfs is that the codebase is much more
42 stable (though ZoL is a newer port of it), and that it has more
43 enterprise-oriented features like ZIL/RAID-Z already implemented.
44 Btrfs has license advantages as far as linux is concerned (it can
45 actually go in the main kernel without a rewrite), and it is a bit
46 more flexible in design and is intended as a general-purpose
47 filesystem.
49 Both are definitely the future of file storage compared to ext4, but
50 they both have a lot of caveats today.
52 What I would love to see though is something more optimized for flash
53 like f2fs, but with the feature-completeness and integrity/snapshot
54 capabilities of btrfs/zfs. A log-based filesystem is COW by its
55 nature, so you just need to add that stuff in.
57 --
58 Rich


Subject Author
Re: [gentoo-user] Safeguarding strategies against SSD data loss Pandu Poluan <pandu@××××××.info>