| Differences between file compression and object
compression. |
Comparison with object compression |
- File compression with the zlib compression library is intended primarily for cold objects and
files.
- File compression with the lz4 compression library is intended primarily for active data.
|
When to use file compression |
Use one of the following commands to compress
or decompress files:- mmchattr
- mmapplypolicy with a MIGRATE COMPRESS rule
|
|
Do not do any of the following operations while an
mmrestorefs command is running:- File compression or decompression with mmchattr or
mmapplypolicy
- mmrestripefs or mmrestripefile
|
Warning |
| OS commands display the decompressed size. |
Reported size of compressed files |
- You can defer file compression or decompression until the file
system is not busy.
- Run mmrestorefs or mmrestorefile to complete the deferred
operation.
|
|
- COMPRESSION indicates whether the file is marked
for compression or decompression.
- illCompressed indicates whether the compression or decompression is
complete or deferred.
|
Indicators of file compression or decompression |
| Writing data to a compressed file decompresses
the affected region of the file. |
|
- You can memory-map a file that is already compressed.
- File compression avoids compressing a memory-mapped file.
|
File compression and memory mapping |
- You can open a compressed file for Direct I/O, but internally
the reads and writes are decompressed.
- File compression avoids compressing a file that is opened for
Direct I/O.
|
File compression and direct I/O |
- Files are decompressed when they are moved out of storage that
is directly managed by IBM
Spectrum Scale.
- When you restore files to the IBM Spectrum
Scale file system, you can run mmrestorefs or
mmrestorefile to recompress the files.
|
Backing up and restoring compressed files |
| File compression supports a File Placement Optimizer (FPO) environment or
horizontal storage pools. |
FPO environment |
| Files that belong to AFM and AFM DR filesets can be compressed and
decompressed. |
AFM environment |
- No compression occurs if the space savings is less than 10 per
cent.
- File compression is not supported in some environments.
- File compression with mmapplypolicy is not supported in Microsoft
Windows.
|
Limitations |