Austin S Hemmelgarn wrote on 2015/11/12 08:04 -0500:
On 2015-11-11 21:15, Qu Wenruo wrote:
Hi Anand,
Nice work.
But I have some small questions about it.
Anand Jain wrote on 2015/11/09 18:56 +0800:
These set of patches provides btrfs hot spare and auto replace support
for you review and comments.
First, here below are the simple example steps to configure the same:
Add a spare device:
btrfs spare add /dev/sde -f
I'm sorry but I didn't quite see the benefit of a spare device.
Aside from what Duncan said (and I happen to agree with him), there is
also the fact that hot-spares are (at least traditionally in most RAID
systems) usually used with RAID5 or RAID6 (or some other parity scheme).
So, to summarize:
1. Hot spares are more useful for most users in global context, and in
that case only if they have more than one filesystem.
2. A pool of hot spares is even more useful.
Agreed, just as Ducan said.
Although only one spare device is supported yet.
3. Assuming whole disk usage (as opposed to partitioning), the hot spare
will have no load on it until it gets used, at which point it will
almost always be in better physical condition than the device it
replaced (which is important for HA systems, in such cases you replace
the disk that failed, and make the new disk a hot spare)
OK, that's also right, if no one is calling btrfs dev scan with a interval.
4. Hot spares are more often used (at least from what I've seen) on
parity based raid systems than raid1.
I'm not familiar with parity based RAID5/6 in btrfs, so I can't say for
sure.
But considering the chunk based RAID feature of btrfs, I think parity
based RAID of BTRFS is not that different from current btrfs RAID1.
Just stripe size difference. hole chunk size(RAID1) vs real stripe size
(btrfs RAID5/6)
And if Btrfs support to specify the number of disks used in raid5/6
chunk allocation, for example only use any 3 devices to allocation raid5
chunk even there are 4 devices, it will be much the same case.
I choose Btrfs Raid1 as an example in my mail just because Btrfs raid1
will only use 2 devices no matter how many devices are in the filesystem.
So I'm very curious of why parity based RAID is often used with hot spare.
Thanks,
Qu
In the rather limited case you outlined, I would probably just use raid1
across all three devices myself (unless they were whole disks and not
individual partitions, in which case I'd use a hot spare), but looking
beyond that at my actual usage of BTRFS (multiple filesystems with
multiple different raid profiles, spread across various disks), hot
spares are definitely useful (although they would be more useful if I
could specify that a given hot spare be used only for a given set of
filesystems).
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