Re: [RFC PATCH v1 0/5] BTRFS hot relocation support

From: Duncan
Date: Mon May 20 2013 - 22:25:12 EST

zwu.kernel posted on Mon, 20 May 2013 23:11:22 +0800 as excerpted:

> The patchset is trying to introduce hot relocation support
> for BTRFS. In hybrid storage environment, when the data in rotating disk
> get hot, it can be relocated to nonrotating disk by BTRFS hot relocation
> support automatically; also, if nonrotating disk ratio exceed its upper
> threshold, the data which get cold can be looked up and relocated to
> rotating disk to make more space in nonrotating disk at first, and then
> the data which get hot will be relocated to nonrotating disk
> automatically.

One advantage of a filesystem implementation, as opposed to bcache or
dmcache, is arguably a corner-case, but it's /my/ corner-case, so...

I run an intr*-less (I guess technically, empty initramfs) monolithic-
kernel boot, using the kernel commandline root= and (formerly) md= and
related logic to choose/assemble/mount root directly from the kernel
command line via bootloader (grub2). Thus, any user-space-required-to-
mount-root is out, since I don't have an initr* and thus no early
userspace. That means both lvm2 and dmcache (AFAIK) are out. I'm not
sure about bcache, but it has other negatives, particularly against btrfs-
raid-1 and I'd guess md/raid-1 as well.

Much like md before it, btrfs, while normally requiring the user-space-
required device-scan to properly handle multiple devices, has kernel-
command-line options that allow direct kernel multi-device assembly
without the help of early-userspace/initr*.

So in-btrfs hot-relocation support building on the existing kernel-
command-line multi-device assembly options would definitely be welcomed
by all us no-initr* folks looking at SSDs but not able to afford them
for /everything/ just yet. =:^)

(That said, even if accepted, your solution's a bit late for my own
current needs, but there's surely going to be others hitting the same
issue in a few kernel cycles when your patches could be mainline btrfs,
and having the option at my next upgrade cycle say a couple years out
would be very nice, indeed. =:^)

Duncan - List replies preferred. No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master." Richard Stallman

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