access to hard drives is "blocked" by writes to a flash drive

[deeptech71]

On 03/02/2013 21:46, Edward Tomasz Napierała wrote:
> Wiadomość napisana przez deeptech71 w dniu 2 mar 2013, o godz. 18:29:
>> When one of my flash drives is being heavily written to; typically by ``svn update'' on /usr/src, located on the flash drive; the following can be said about filesystem behavior:
>>
>> - ``svn update'' seems to be able to quickly update a bunch of files, but is then unable to continue for a period of time. This behavior is cyclical, and cycles several times, depending on the amount of updating work to be done for a particular run of ``svn update''.
>> - Access to any filesystem, whether on the said flash drive or not, seems to be hindered a lot, typically during the said "unable to continue" time. Reading of a file on a hard drive was once delayed for more than 10 seconds. Seemingly as a consequence, the starting of top(1) is also typically delayed.
>
> When that happens, could you do "ps axl" and see the WCHAN column
> for the affected processes?  Is it "wdrain"?

What do you mean by "the affected processes"?

The MWCHAN of an ``svn update'' contained long "wdrain" periods (but included "getblk", "drainvp", etc.). ``[bufdaemon]'' sometimes seemed to somewhat follow the "wdrain" state of ``svn update''.

I've also found out that the inability for top(1) or ps(1) to start immediately has something to do with X -- in the virtual terminals, there was never a delay.