SSD for boot and swapcache
dillon at apollo.backplane.com
Fri Dec 3 13:15:19 PST 2010
I also have an explanation in the swapcache manual page:
But since people keep asking about it I'll try to explain it more
fully. Hmm. I might as well add this to manual page too, which
I will do.
Modern SSDs keep track of space that has never been written to.
This would also include space freed up via TRIM, but simply not
touching a bit of storage in a factory fresh SSD works just as well.
Once you touch (write to) the storage all bets are off, even if
you reformat/repartition later. It takes sending the SSD a
whole-device TRIM command or special format command to take it back
to its factory-fresh condition (sans wear already present).
SSDs have wear leveling algorithms which are responsible for trying
to even out the erase/write cycles across all flash cells in the
storage. The better a job the SSD can do the longer the SSD will
The more unused storage there is from the SSDs point of view the
easier a time the SSD has running its wear leveling algorithms.
Basically the wear leveling algorithm in a modern SSD (say Intel or OCZ)
uses a combination of static and dynamic leveling. Static is the
best, allowing the SSD to reuse flash cells that have not been
erased very much by moving static (unchanging) data out of them and
into other cells that have more wear. Dynamic wear level is
the worst kind but the easiest to implement. USB sticks often use
only dynamic wear leveling and have short life spans because of
that. SSDs which do write-combining (placing non-linear sectors
next to each other physically in order to reduce write amplification
effects) also use static wear leveling to simplify write-combined areas,
making accesses to the SSD go faster by reducing the complexity of
the lookup tables the SSD has to maintain.
In anycase, any unused space in the SSD effectively makes the dynamic
wear leveling the SSD does more efficient by giving the SSD more 'unused'
space above and beyond the physical space it reserves beyond its stated
storage capacity to cycle data throgh, so the SSD lasts longer in theory.
It's very hard to test this sort of thing. I've been running a long
term test on one of our package building boxes for almost a year now
with a 40G Intel SSD using the space reservation idea (using only 32G
out of a 40G SSD) but I don't have a similar test using the whole
drive so I can't really determine how much more efficient it is. But
I still recommend it. Any drive which has TRIM will definitely make
use of any pristine unused storage just as if it had been TRIMmed.
<dillon at backplane.com>
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