Re: [9fans] fossil pb: FOUND!

[erik quanstrom <quanstro@quanstro.net>]

On Sat Jan 14 17:12:49 EST 2012, aram.h@mgk.ro wrote:
> > content addressed means given the content, you can generate the address.
> > this is NOT true of zfs at all.
>
> How come? With venti, the address is the SHA-1 hash, with ZFS, you get
> to chose the hash, but it can still be a hash.

because in zfs the hash is not used as an address (lba).

> My original claim was that fragmentation is a non issue if you have
> SSDs.  I still claim this and I expanded on the context in my previous
> post.  Of course that random I/O is slower than sequential I/O, SSD or
> not, but in practice, filesystem fragmentation causes an amount or
> random I/O much less than what a SSD can handle, so throughput in the
> fragmented case is close to the throughput in the sequential case.
>
> I don't think that caching is completely irrelevant.  If I have to
> chose between a complex scheme that avoids fragmentation and a simple
> caching scheme that renders it irrelevant for a particular workload,
> I'll chose the caching scheme because it's simpler.

by all means, show us the numbers.  personally, i believe the mfgrs are not
lying when they say that random i/o yields 1/10th the performance (at best)
of sequential i/o.

since you assert that it is ssds that make random i/o a non issue, and
not caching, logically caching is not relevant to your point.

- erik

ps.  you're presenting a false choice between caching and fragmentation.
case in point, ken's fs doesn't fragment as much as venti (and does not
increase fragmentation over time) and yet it caches.