Re: [f2fs-dev] f2fs compressed file bio merge problem

[Eric Biggers <ebiggers@kernel.org>]

[+Cc linux-fscrypt]

On Mon, Apr 18, 2022 at 08:15:47AM +0000, 常凤楠 wrote:
> Hi:
> 	When I test seq-write on f2fs compressed file, I found it may have significant performance degradation when mount with inlinecrypt. Here is my analysis: 	
> 	f2fs write compressed file in unit of cluster, after compressed, one cluster up to have three valid pages to write. So between multi clusters, the page index couldn't be contiguous. For example, It may like this: Cluster 0 write page 0 and 1, Cluster 1 write page 4 and 5.
> 	In f2fs_crypt_mergeable_bio, fscrypt_mergeable_bio will check weather file logical block number is contiguous, result in multi clusters cannot be merge into one bio.
> 	In my test, inlinecrypt mount option may cause seq-write performance to drop by half.
> 	The attachment is my fio test configure file.
> 	This is a tricky problem for me. Is there any solution for this problem?

Thanks for clarifying that you are using f2fs compression; in your previous
message you didn't mention this
(https://lore.kernel.org/all/KL1PR0601MB4003998B841513BCAA386ADEBBEE9@KL1PR0601MB4003.apcprd06.prod.outlook.com/T/#u).

Unfortunately, I don't believe there is any practical way that we could do the
encryption differently that would result in this no longer being a problem.

This is because for adjacent clusters to have contiguous DUNs, the DUNs would
have to increment according to the compressed size, not the uncompressed size.
However, in this case it wouldn't be possible to support random-access writes,
since any write would require re-writing the entire file.

This could be provided as an option for read-only filesystems, I suppose.  But I
doubt that that is your use case.

- Eric


_______________________________________________
Linux-f2fs-devel mailing list
Linux-f2fs-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel