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Fwd: Re: Server Hardware

  I just saw the attached email message on a LUG mailing list.  It's the first 
time I've heard of this, and it looks a bit worrying, especially since a lot 
of newer machines are shipping SATA-only these days.

  What I'm wondering is:

   (a) Does the default sarge kernel run afoul of the problems alluded to in 
this email?  I suppose the issue would be that you don't get the usual 
benefits of journalling.

  (b) If it does, is there a big fat warning somewhere in the installer when 
the user tries to partition a SATA disk with a journalling filesystem?

  Daniel

-- 
/----------------- Daniel Burrows <d.burrows4@verizon.net> -----------------\
|               If we do not change our direction                           |
|               we are likely to end up where we are headed.                |
\------ Listener-supported public radio -- NPR -- http://www.npr.org -------/
--- Begin Message --- 
On Wed, 23 Mar 2005 21:23:14 -0500, Jeff Wolfe <wolfe@ems.psu.edu> wrote:
> James Gingerich wrote:
> > Hello All,
> >
> > I'm getting ready to make a purchasing decision with limited funds and
> > would like some advice.
> >
> > My department is looking to purchase a new tower server, running Linux
> > of course.  Its main purpose is a web server (Apache front end, Tomcat
> > application server), but also uses its own RDBMS, and a little mail and
> > file sharing (samba).  It backs up regularly, with little changes.
> >
> > I'm trying to determine what the best comprimise of hardware for my cost
> > is.  I believe the key items are processor, memory, and hard drive(s).
> > I just don't know which is more important for the server to squeeze the
> > best performance, i.e.; weighing 2 processors vs. more memory vs. SCSI
> > drives?
> >
> > Here is what I come up with thus far:
> >
> > 1 Xeon processor @ 3.0 GHz, 2 MB Cache, 800 MHz FSB
> > 2 GB memory DDR2 @ 400 MHz
> > CERC SATA RAID controller (RAID 0)
> > 2 160 GB SATA hard drives
> >
> > Any suggestions would be greatly appreciated.
>
> I would encourage you to mirror the drives instead of striping.
>
> Also, if you're thinking of a distribution other than Redhat. There sometimes
> can be issues with drivers and non-redhat distros.. Dell only certifies their
> systems with RedHat.
>

Depending on how much you care about data integrity, things get more
complicated.  SATA disks use write-back cache in order to provide
performance that is competitive with SCSI disks in write-through mode.
 Unfortunately, the sata protocol state machine assumes write-back is
acceptable, and thus does not implement the scsi tcq asynchronous
callback model.  The net effect is that transactional systems, such as
journalling filesystems and databases, will not operate correctly on
sata disks, unless those transactional layers have special sata
extensions.  Recent 2.6 kernels support journalling on sata through
their write barriers interface, which is just a cache flush command on
the backend.  So, if you care about integrity, your options are to
turn off disk write caches (which makes performance abysmal due to
sata's poorly designed state machine), use a very recent 2.6 kernel,
or use a distro that has write barriers back-ported into their custom
kernel.  Furthermore, you need to be sure that you get the midrange
sata disks, and not the desktop-oriented disks, because the MTBFs are
very different.

--
Tom Keiser
tkeiser@gmail.com

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