dar backup and atop
I have a script using dar to make disk backups.
I noticed with top/atop that a process pdflush was being busy, which I found out was to be expected.
(about pdflush, see: http://lwn.net/Articles/326552/ )
However, I also found this:
Tuning Recommendations for write-heavy operations
The usual issue that people who are writing heavily encouter is that Linux buffers too much information at once, in its attempt to improve efficiency. This is particularly troublesome for operations that require synchronizing the filesystem using system calls like fsync. If there is a lot of data in the buffer cace when this call is made, the system can freeze for quite some time to process the sync.
Another common issue is that because so much must be written before any phyiscal writes start, the I/O appears more bursty than would seem optimal. You'll have long periods where no physical writes happen at all, as the large page cache is filled, followed by writes at the highest speed the device can achieve once one of the pdflush triggers is tripped.
-> Check how dar is performing in that respect.
-> what kind of finetuning would be advisable?
A related question:
dar eats a lot of my plentyfull RAM, flushing web applications that had been idle into the swap and that bothers me.
How to limit the amount of RAM that dar can consume?
HOWTO: Decrease disk activity