On Jan 11, 2010, at 2:23 PM, Bob Friesenhahn <bfrie...@simple.dallas.tx.us
> wrote:
On Mon, 11 Jan 2010, bank kus wrote:
Are we still trying to solve the starvation problem?
I would argue the disk I/O model is fundamentally broken on Solaris
if there is no fair I/O scheduling between multiple read sources
until that is fixed individual I_am_systemstalled_while_doing_xyz
problems will crop up. Started a new thread focussing on just this
problem.
While I will readily agree that zfs has a I/O read starvation
problem (which has been discussed here many times before), I doubt
that it is due to the reasons you are thinking.
A true fair I/O scheduling model would severely hinder overall
throughput in the same way that true real-time task scheduling
cripples throughput. ZFS is very much based on its ARC model. ZFS
is designed for maximum throughput with minimum disk accesses in
server systems. Most reads and writes are to and from its ARC.
Systems with sufficient memory hardly ever do a read from disk and
so you will only see writes occuring in 'zpool iostat'.
The most common complaint is read stalls while zfs writes its
transaction group, but zfs may write this data up to 30 seconds
after the application requested the write, and the application might
not even be running any more.
Maybe an IO scheduler like Linux's 'deadline' IO scheduler whose only
purpose is to reduce the effect of writers starving readers while
providing some form of guaranteed latency.
-Ross
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