Re: Creating compressed backing_store as swapfile

[Austin S. Hemmelgarn]

On 11/5/2018 10:58 AM, Adam Borowski wrote:
> On Mon, Nov 05, 2018 at 08:31:46PM +0530, Pintu Agarwal wrote:
> > Hi,
> >
> > I have one requirement:
> > I wanted to have a swapfile (64MB to 256MB) on my system.
> > But I wanted the data to be compressed and stored on the disk in my swapfile.
> > [Similar to zram, but compressed data should be moved to disk, instead of RAM].
> >
> > Note: I wanted to optimize RAM space, so performance is not important
> > right now for our requirement.
> >
> > So, what are the options available, to perform this in 4.x kernel version.
> > My Kernel: 4.9.x
> > Board: any - (arm64 mostly).
> >
> > As I know, following are the choices:
> > 1) ZRAM: But it compresses and store data in RAM itself
> > 2) frontswap + zswap : Didn't explore much on this, not sure if this
> > is helpful for our case.
> > 3) Manually creating swapfile: but how to compress it ?
> > 4) Any other options ?
>
> Loop device on any filesystem that can compress (such as btrfs)?  The
> performance would suck, though -- besides the indirection of loop, btrfs
> compresses in blocks of 128KB while swap wants 4KB writes.  Other similar
> option is qemu-nbd -- it can use compressed disk images and expose them to a
> (local) nbd client.

Swap on any type of a networked storage device (NBD, iSCSI, ATAoE, etc) 
served from the local system is _really_ risky.  The moment the local 
server process for the storage device gets forced out to swap, you deadlock.

Performance isn't _too_ bad for the BTRFS case though (I've actually 
tested this before), just make sure you disable direct I/O mode on the 
loop device, otherwise you run the risk of data corruption.