[hawski]

Oh, gosh. Since when it's without a maintainer? I would think it's quite an important piece of most embedded Linux projects. Those companies really should sponsor someone to take care of it.

My personal project also depends on squashfs. Are there any other options for read-only compressed rootfs?

Squashfs-tools are a bit rough around the edges that's for sure. I patched in ability to produce an image without root for my use, but I think it would be generally useful. Sort list file format is funky. It's: "filename_string priority_integer", so it will not work for files with spaces. Also new lines, but that's far less common. It was not yet a problem for me and I can always patch it more and one can say, that people having files with spaces deserve it, though it's entirely different issue.

[cmurf]

Btrfs supports compression. A "seed" flag that makes it read-only. If you 'btrfs device add' a rw second device, it can be read-write with all writes directed to the second device, e.g. /dev/zram device for volatile overlay for e.g. a LiveOS boot, or you can add a blank partition and then remove the seed which causes the seed data to be replicated to the rw partition. Plus all metadata and data are checksummed.

zstd support since linux 4.14, and a mount time option for compression level since linux 5.1. So you could get very good compression ratios equivalent to squashfs, but squashfs will still come out slightly ahead because it also compresses its own metadata, where Btrfs doesn't.

My best guess is squashfs is a sufficiently successful project that it's allowed most distributions who depend on it (quite a few) to sit on its laurels.

[tych0]

Looks like he might (?) be coming back: https://github.com/plougher/squashfs-tools/issues/54#issueco... although the release he's talking about didn't happen.

The last commit that wasn't an API refactoring in the kernel tree is a3f94cb99a854fa381fe7fadd97c4f61633717a5, which is from Aug 2, 2018.

[plougher]

It is not without a maintainer. But you make a good point that the companies that use it should sponsor it. I still have to maintain it for free in my spare time, and spare time gets less and less each year.

In fact it woudn't surprise me if the parent commenter is one of the people working for a multi-billion dollar company that got offended when I told them I wouldn't do some work for them, for free. Hence the suspicion there is an axe to grind.

[usr1106]

squashfs has horrible performance. All requests to the block layer are 512 Bytes. Other filesystems like ext4 make much bigger requests and perform much better in the end despite the compression of squashfs leading to lower overall data volume.

Disclaimer: Measured 2 years ago on ARM32, emmc, with a 4.1(?) kernel.

[plougher]

Try using the SQUASHFS_4K_DEVBLK_SIZE config option next time

By default Squashfs sets the dev block size (sb_min_blocksize) to 1K or the smallest block size supported by the block device (if larger). This, because blocks are packed together and unaligned in Squashfs, should reduce latency.

This, however, gives poor performance on MTD NAND devices where the optimal I/O size is 4K (even though the devices can support smaller block sizes).

Using a 4K device block size may also improve overall I/O performance for some file access patterns (e.g. sequential accesses of files in filesystem order) on all media.

Setting this option will force Squashfs to use a 4K device block size by default.

If unsure, say N.

[usr1106]

I'm quite sure I have tried all options available in the kernel I used back then without achieving performance comparable to ext4. My project manager was conviced that squashfs makes things faster (and so hoped I initially because the overall data volume is smaller) so I had a hard time to convince him that we will just drop that "optimization" from the project plan. (He was one of those who can prefer checkmarks over technical merit.) I don't remember the 4K option for sure, but if it existed, we tried and measured it. What is the size ext4 is reading from the block device? I'm reading this on holidays on my phone, so I cannot easily fire up blktrace. But I could guess it's 128K or even 256K. So still far from 4K.

[usr1106]

emmc is not a MTD device. I measured only om eMMC.

[cmurf]

A big part of the problem might be xz decompression, that's been my discovery anyway.

https://bugzilla.redhat.com/show_bug.cgi?id=1717728

[usr1106]

I tried various compression algorithms. I don't think there was a CPU bottleneck, at least not with the less aggressive compressions. blktrace showed the difference compared to ext4, squashfs does all reads one by one block.

It was in a previous job. I don't have access to the details anymore. And the kernel was not the newest. But squashfs looked unmaintained already then and that's what they're saying elsewhere in the discussion. So I fear nothing has changed.