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Linux Storage and Filesystem workshop, day 1

Apr 24, 2009, 19:02 (0 Talkback[s])
(Other stories by Jonathan Corbet)

"Things began with a quick recap of the action items from the previous year. Some of these had been fairly well resolved over that time; these include power management, support for object storage devices, fibre channel over Ethernet, barriers on by default in ext4, the fallocate() system call, and enabling relatime by default. The record for some other objectives is not quite so good; low-level error handling is still not what it could be, "too much work" has been done with I/O bandwidth controllers while nothing has made it upstream, the union filesystem problem has not been solved, etc. As a whole, a lot has been done, but a lot remains to do.

"Device discovery

"Joel Becker and Kay Sievers led a session on device discovery. On a contemporary system, device numbers are not stable across reboots, and neither are device names. So anything in the system which must work with block devices and filesystems must somehow find the relevant device first. Currently, that is being done by scanning through all of the devices on the system. That works reasonably well on a laptop, but it is a real problem on systems with huge numbers of block devices. There are stories of large systems taking hours to boot, with the bulk of that time being spent scanning (repeatedly - once for every mount request) through known devices.

"What comes out of the discussion, of course, is that user space needs a better way to locate devices. A given program may be searching for a specific filesystem label, UUID, or something else; a good search API would support all of these modes and more. What would be best would be to build some sort of database where each new device is added at discovery time. As additional information becomes available (when a filesystem label is found, for example), it is added to the database. Then, when a specific search is done, the information has already been gathered and a scan of the system's devices is no longer necessary."

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