Re: high load, no bottleneck

[christos%zoulas.com@localhost (Christos Zoulas)]

On Sep 17,  9:48pm, manu%netbsd.org@localhost (Emmanuel Dreyfus) wrote:
-- Subject: Re: high load, no bottleneck

| Excellent: the load does not go over 2 now (compared to 50).
| 
| Thank you for saving my day. But now what happens?
| I note the SATA disks are in IDE emulation mode, and not AHCI. This is
| something I need to try changing:

What happens highly depends on the drive (how frequently it flushes
cache to disk internally and how long does it keep data in-cache),
but it is never good. The best case scenario is would be that WAPBL
writes are ordered properly and that cache-flush is only send
occasionally between transactionally safe metadata commit points, but
it seems that this is not happening (because we are getting too many
flushes).

The case to worry about is the scenario where the machine
suddently loses power, the data never makes it to the physical media,
and gets lost from the cache. In this case you might end up with a
filesystem that has inconsistent metadata, so the next reboot might
end up causing a panic when the filesystem is used. The solution there
is to reboot and force an fsck. If you have a UPS I would not worry
too much about it; even if your system panics the kernel should issue
the flush commands to the disk.

BTW I hope that everyone realizes that WAPBL deals only with metadata
and not the actual file data, so if you crash/lose power you typically
end up with garbage in the active files (usually bits and pieces of files
form other files, or NUL's).

christos