JFS compression

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If a file system is compressed, all data is compressed automatically using Lempel-Zev (LZ) compression before being written to disk, and all data is uncompressed automatically when read from disk. The LZ algorithm replaces subsequent occurrences of a given string with a pointer to the first occurrence. On an average, a 50 percent savings in disk space is realized.

File system data is compressed at the level of an individual logical block. To compress data in large units (all the logical blocks of a file together, for example) would result in the loss of more available disk space. By individually compressing a file's logical blocks, random seeks and updates are carried out much more rapidly.

When a file is written into a file system for which compression is specified, the compression algorithm compresses the data 4096 bytes (a page) at a time, and the compressed data is then written in the minimum necessary number of contiguous fragments. Obviously, if the fragment size of the file system is 4 KB, there is no disk-space payback for the effort of compressing the data. Therefore, compression requires fragmentation to be used, with a fragment size smaller than 4096.

Although compression should result in conserving space overall, there are valid reasons for leaving some unused space in the file system:

  • Because the degree to which each 4096-byte block of data will compress is not known in advance, the file system initially reserves a full block of space. The unneeded fragments are released after compression, but the conservative initial allocation policy may lead to premature "out of space" indications.
  • Some free space is necessary to allow the defragfs command to operate.

In addition to increased disk I/O activity and free-space fragmentation problems, file systems using data compression have the following performance considerations:

  • Degradation in file system usability arising as a direct result of the data compression/decompression activity. If the time to compress and decompress data is quite lengthy, it might not always be possible to use a compressed file system, particularly in a busy commercial environment where data needs to be available immediately.
  • All logical blocks in a compressed file system, when modified for the first time, will be allocated 4096 bytes of disk space, and this space will subsequently be reallocated when the logical block is written to disk. Performance costs are associated with reallocation, which does not occur in noncompressed file systems.
  • To perform data compression, approximately 50 CPU cycles per byte are required, and about 10 CPU cycles per byte for decompression. Data compression therefore places a load on the processor by increasing the number of processor cycles.
  • The JFS compression kproc (jfsc) runs at a fixed priority of 30 so that while compression is occurring, the CPU that this kproc is running on may not be available to other processes unless they run at a better priority.