Read-only FS compression

[wkozaczuk]

Current implementation of ROFS does not compress files. Compressing files should make ROFS images smaller and possibly speed up read operation as less data would need to be read from block device.

ROFS compression could use same algorithm as ZFS does - LZ4. Potential implementation could be based on compressing/decompressing files around same boundaries used in "read-around" strategy applied in ROFS caching layer. We could compress each 32K file cache segment and store size of compressed segment as number of 512-bytes blocks in file i-node metadata so that we can load correct data to decompress when particular segment is missed when reading - look here.