On 10/04/2020 11:59 AM, Pavel Machek wrote:
Hi!
In particular, using this functionality, user is able to push out
"hot" files on any high-performance device (e.g. proxy device) and pin
them there.
What permissions are normally required for file migration?
Hi Pavel,
I guess, admin ones.
With such operation it is possible to organize an attack on a
collectively shared volume by clogging some its brick. So that other
users, who rely on regular distribution (provided by per-volume
distribution table) will get "no space left on device", while other
bricks contain a lot of free space..
COMMENT. After ioctl successful completion the file is not necessarily
written to the target device! To make sure of it, call fsync(2) after
successful ioctl completion, or open the file with O_SYNC flag before
migration.
Ok.
COMMENT. File migration is a volume operation (like adding, removing a device to/from
a logical volumes), and all volume operations are serialized. So, any attempt to
migrate a file, while performing other operation on that volume will fail. If some
file migration procedure fails (with EBUSY, or other errors), or was interrupted by
user, then it should be repeated in the current mount session. File migration
procedures interrupted by system crash, hared reset, etc) should be repeated in the
next mount sessions.
Dunno. Returning -EBUSY is kind of "interesting" there. I'd expect kernel to queue
the callers, because userland can't really do that easily.
You are right. The current solution is temporary. Actually, we don't
need to lock the whole volume in order to migrate a file (anyway, the
file migration procedure takes an exclusive access to the file).
User-defined migration of individual files should be serialized with
brick removal. So it will be even per-brick lock rather than per-volume
lock.. I think, that it should be a rw-semaphore. Brick removal
procedure will take a write lock (with possible waiting) and
user-defined migration will try to take a read lock. If busy, then
return error (brick is under removal == doesn't exist for user).
Thanks,
Edward.
