In defense of per-BDI writeback
Posted Oct 3, 2009 15:00 UTC (Sat) by anton
Parent article: In defense of per-BDI writeback
The fundamental problem with pdflush is that it
would back off when the backing device appeared to be congested.
That might explain the huge slowdowns I saw (on Linux 2.6.19 and
2.6.27) when writing several GB to flash devices. One was a pretty
fast 8GB SD card (SDHC class 6 (i.e., >6MB/s on a certain workload),
and I typically saw >10MB/s when writing several hundred MB), yet it
took several hours to fill up; I no longer remember if the system also
suffered in other ways. Calling sync now and then seemed to help,
but the whole thing still took a very long time.
I do not think that the problem was in the flash device, because it
was originally new (no need to shuffle old data around), the slowdown
occured pretty soon (not only near the end), and various measures
taken at the host end helped (like invoking sync, or writing the data in
smaller batches which syncing in between).
I had a similar experience when trying to fill my 8GB ogg player with
music, except that this device was slow to begin with (3MB/s when
writing a few hundred MB), but filling it up still should not have
taken 8 hours (280KB/s).
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