SquashFS is a compressed read-only filesystem for Linux. The file system is read-only by design and thus suitable for use on the system partition. Many Android devices may benefit from using this file system for their system partition, for example, the following:

• Devices with a low capacity storage such as Android Watch.
• Devices with a slow flash storage (compression reduces the number of block I/Os).

Unfortunately the performance of SquashFS lags behind ext4.

Optimizations

The following optimizations have been implemented to improve the performance of SquashFS.

Reduce the memory usage and memcpy

When reading a block (default 128K), SquashFS tries to grab all the pages covering this block.

If a single page is up-to-date or locked, it falls back to allocating a full block, submitting a read request, and then copying its content to the pages.

This approach is very ineffective; after some time the page cache is likely to contain pages that are up-to-date even if the adjacent pages are not.

The code is now able to handle blocks with holes (=missing pages). This improves performance in the following ways:

• Reduces the number of memcpy calls
• Decreases memory allocations

Asynchronous reads

SquashFS still uses the deprecated ll_rw_block() function. There are two problems with this approach:

• As the name implies, the function waits for the reads to complete before returning. This is redundant since .readpage() already waits on the page's lock. Moreover, we need an asynchronous mechanism to efficiently implement .readpages().
• Merging the read requests entirely depends on the I/O scheduler. ll_rw_block() simply creates one request per buffer. SquashFS has more information than the I/O scheduler about what should be merged. Moreover, merging the request means that we rely less on the I/O scheduler.

For these reasons, the ll_rw_block() function has been replaced with submit_bio().

Readpages (prefetching)

SquashFS does not implement .readpages(), so the kernel repeatedly calls .readpage().

Now that our read requests are asynchronous, the kernel can truly prefetch pages using its asynchronous read-ahead mechanism.

Optimize reading uncompressed blocks

Modern systems such as Android contain a lot of files that are already compressed. As a consequence, the image contains a lot of blocks that can't be compressed.

SquashFS handles compressed and uncompressed blocks using the same logic: when asked to read a single page, it actually reads a full block (default 128k). While this is necessary for compressed blocks, it is just a waste of resources for uncompressed blocks.

Instead of reading a full block, SquashFS now just reads what is advised by the readahead algorithm.

This greatly improves the performance of random reads.

Code

SquashFS code optimizations are available in AOSP:

• https://android-review.googlesource.com/#/q/topic:squashfs