Mailinglist Archive: opensuse (2417 mails)

[Sam Clemens <clemens.sam1@xxxxxxxxx>]

Neil wrote:
> On Sun, Apr 6, 2008 at 12:54 PM, Sam Clemens <clemens.sam1@xxxxxxxxx> wrote:
> > Sudhir wrote:
> >
> > > Patrick Shanahan wrote:
> > >
> > > > * JB2 <yonaton@xxxxxxxxxxxx> [04-05-08 22:21]:
> > > >
> > > >
> > > > >  The new hdd is SATA. Will it mess anything up to have an SATA drive
> > and an ide drive on my system like this?
> > > > >
> > > > I have five ide drives and three sata drives in this 10.1 system.
> > > I am green with envy :).
> >  It's not hard.  Just never throw a drive away until it's
> >  dead.
> >  Personally, I always use my oldest drive or drives (depending
> >  on size and space requirements) for /tmp and swap space.
> >
> >  Yes, I know swap SHOULD be a fast device, not some old,
> >  device from 2 technology generations ago...but I'll put
> >  up with a bit of a speed reduction if it means that I'm
> >  NOT beating the hell out of my fastest (i.e. NEWEST)
> >  disk drives by putting swap on any of them....because
> >  the newer the drive, the more important the data on them.
> >
> >  Newest drives           filesystem
> >
> >  Newest drive(s):        /home, /www, and other content created
> >                                 by me or other users of the system
> >
> >                         /local
> >
> >                         /opt    some things here require more
> >                                 then just an OS re-install to
> >                                 create again.
> >
> >                         rest of operating system
> >
> >  Oldest drive(s):        /tmp, swap space
> >
> >  How do I do this?  Easy.
> >
> >  I never buy expensive, huge drives:  Why buy a 500 GB
> >  drive if I only have 50 GB of data right now??  I can
> >  buy a 200 GB drive now for about $45...and by the time
> >  I fill that thing up, for another $50 or so, I'll probably
> >  get something even bigger than 300 GB.
> >
> >  Or, I could spend much more than $95 now to buy the
> >  500GB drive all at once.
>
> If I am correct and swap space is striped over the different drives it
> is placed upon then it should matter more how many drives you have
> with swap spaces on it.

Swap space is striped ONLY if you set it up to be so.
If two swap spaces have different priority levels, one
swap space will be exhausted before the system starts
to use the 2nd one.

> True: the access time of old harddisks is
> worse, but then again: the swap space will probably be used to store
> the bigger files anyways,

Swapping is not used to "store files"... it's used to
page out memory pages so that the CPU can use a larger
address space than the amount of physical memory
installed (or more specifically:
virtual address space = physical mem + swap space)

[This ignores annoying things like memory-mapped I/O, etc.]


> since Linux is smart in handeling swap
> space.

smart, yes.
magic: no

> The access time of a harddisk is always a order of magnitude
> longer than the acces time of RAM, even with the newest (non solid
> state) disks, so IF the designers of the daemons that handle the swap
> space of Linux are as smart as I believe they are then (when swapspace
> is needed) the largest files will have a larger chance of getting in
> the swapspace (although there most likely are strong arguments why
> some files shoudn't be swapped if possible).

But swapping isn't done by file, it's done on a page by page
basis.  You can have some of a file in memory, some of it
swapped out, and some of it not even read yet.

And pages to be swapped are evaluated on an individual
basis (what's the current page size now...4k? 16k? 54k?)

Google for "demand-paged-virtual-memory", and
take a look at these:
<http://en.wikipedia.org/wiki/Paging>
<http://en.wikipedia.org/wiki/Page_replacement_algorithm>
<http://en.wikipedia.org/wiki/Demand_paging>

Note that Windows has historically NOT used "demand
paging"...instead, what they call "aggressive paging"
(anticipatory paging also known as prepaging) which
in theory works great, but in practice, results in
lots of paging activity which is never needed -- it
seems to be one of the primary reasons why Windows is
so pathatically non-responsive so often -- instead of
just paging out what needs to be paged out, and then
getting with running your program ... n ooooooooo, it
decides to page out more memory, so that it can
pre-load code within a branch that never gets
executed. [Great plan you got there, Redmondites].


> So: the resulting speed of your swap space is probably in lesser
> extend a result of the actual speed of the harddisk and to greater
> extend a result of the number of harddisks in swap.
>
> I would have used my new drives for swap space, but you started a
> train of thougts causing me to reconsider that.Hmm, if I simply use
> the tower of 1 to 5 G 5400 rpm harddisks to create swap space that
> might save me from a crash in the future.

If your swap drive dies, you're still going to crash.

But if there's nothing else on the disk, at least you're
not losing filesystems, too.


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