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Subject: Re: Creating compressed backing_store as swapfile
To:     Adam Borowski <kilobyte@angband.pl>,
        Pintu Agarwal <pintu.ping@gmail.com>
Cc:     linux-mm@kvack.org, open list <linux-kernel@vger.kernel.org>,
        kernelnewbies@kernelnewbies.org
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From:   "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
Message-ID: <79d0c96a-a0a2-63ec-db91-42fd349d50c1@gmail.com>
Date:   Mon, 5 Nov 2018 11:07:12 -0500
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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.