Benutzer:Rdiez/Linux zram

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Linux zram

zram is a Linux block device that lives in RAM and compresses any data written to it.

When created, a zram device advertises its nominal capacity as empty space but allocates no actual memory. You can then create a filesystem on the zram device and use it as a RAM disk. As data is gradually written to it, zram will allocate as much kernel memory as necessary to hold the compressed data.

Compressing data takes CPU time, but can be faster than writing to a physical hard disk. If the data happens to not compress well, zram will allocate as much RAM as all the original data. If that is always the case, then it is best to use an uncompressed RAM disk like ramdisk, ramfs or tmpfs.

zram does not support the TRIM command, which is typically used with SSD disks. Therefore, zram will not free allocated memory even if all files are deleted. It's a little weird, because zram's predecessor project compcache does have this feature.

Availability

zram was considered stable as of Linux kernel version 3.14, released in March 2014. Ubuntu 14.04 has kernel version 3.13 and its zram is still considered experimental, although it had already enjoyed widespread usage then.

The version planned for kernel 3.15 supports the alternative compression algorithm LZ4, in addition to the default LZO. zram does NOT attempt to recompress the data in the background if the CPU is idle.

Using zram as Swap Device

Using zram as a swap device is an interesting idea. I have not found on the Internet many performance tests yet, but most people who have tried it report an increased performance for standard office tasks, especially if the computer has little RAM. That seems like a contradiction at first, because zram uses RAM itself, making a scarce resource even scarcer.

A typical compression ratio for a zram swap file, taking all the management overhead into consideration, is factor 2.5. This means that "normal" swap data gets compressed down to a 40% of its original size. Therefore, for every byte of RAM that zram takes away, you get 2.5 bytes of a very fast swap file, at least compared to a traditional hard disk. You will also lose some CPU performance during the data compression.

Usually, one zram block device (with its corresponding swap file) is created per CPU core, in an attempt at distributing the CPU load. By the way, I heard that a future version will support multiple threads on a single device. All those swap files will have a higher priority than any other hard disk-based swap areas, so Linux will swap to zram first, and continue swapping to disk only if zram fills up.

If you have an SSD disk or similar storage that needs wear leveling, zram may not offer a performance advantage. On the other hand, swapping to RAM first may avoid constant writes to the SSD and help prolong its lifetime.

Note that zram can be used in embedded devices without any storage device for a standard swap file. This way, the virtual memory size can be somewhat larger than the RAM size.

Installing and Removing zram

TODO

Alternative

zswap takes the cache approach instead, so it always needs a swap device underneath. It is more flexible, as it can dynamically cache any part of the swap file, no matter how much RAM the computer has. Ubuntu's 14.04 stock kernel does not support zswap.