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SubjectRe: Creating compressed backing_store as swapfile
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On 11/5/2018 11:53 AM, valdis.kletnieks@vt.edu wrote:
> On Mon, 05 Nov 2018 11:28:49 -0500, "Austin S. Hemmelgarn" said:
>
>> Also, it's probably worth noting that BTRFS doesn't need to decompress
>> the entire file to read or write blocks in the middle, it splits the
>> file into 128k blocks and compresses each of those independent of the
>> others, so it can just decompress the 128k block that holds the actual
>> block that's needed.
>
> Presumably it does something sane with block allocation for the now-compressed
> 128K that's presumably much smaller. Also, that limits the damage from writing to
> the middle of a compression unit....
>
> That *does* however increase the memory requirement - you can OOM or
> deadlock if your read/write from the swap needs an additional 128K for the
> compression buffer at an inconvenient time...
>
Indeed, and I can't really comment on how it might behave under those
circumstances (the systems I did the testing on never saw memory
pressure quite _that_ bad, and I had them set up to swap things out
pretty early and really aggressively).

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