RE: Solid State Drives

From: Tanel Poder <>
Date: Fri, 1 May 2009 21:33:57 +0300
Message-ID: <5361F4E46FB14B2ABE64DF1AED9CDA5F_at_porgand>

Well even without wear levelling and copy-on-writes, assuming you have loaded the SSD 100% full with redo logs only then you could write into this redolog space many times.

So if we do an exercise assuming that:

  1. you have 8 x 1 GB redologs for a database on a SSD
  2. it takes 1 hour to fill these 8 GB of logs ( 8 GB per minute = 192 GB per 24 hours ) - you will be writing to the same block once per an hour (two times to redo header though as its updated when it gets full to mark the end SCN in the file)
  3. its possible to write to the SSD disk block for "only" 100,000 times

So, if you write to a block max 2 times per hour it would still mean 50,000 hours it would still be over 5 years.

The controlfiles and temp tablespace files (and sometimes undo) experience much more writes to "same" blocks compared to redologs, they would be the first ones hitting problems :)

But there IS the write levelling which avoids writing to the same blocks too much by physically writing somewhere else and updating the virtual/physical location translation table. Much depends on the algorithm used...

Regarding whether the mirrored SSDs would wear out at the same time - probably not as the number of writes before wearing out is not some fixed discrete number it will probably vary quite a lot. And you would not want to wait until these disks fail anyway, but rather replace them before known "expire time". And this expire time would be measured in number of write operations rather than wall clock time.

Tanel Poder

> -----Original Message-----
> From:
> [] On Behalf Of Jeremy Schneider
> Sent: 01 May 2009 19:24
> To:
> Cc:;;
> Subject: Re: Solid State Drives
> Matthew Zito wrote:
> >
> > As far as the upgrade path, the lifespan is comparable for
> a "spinning
> > rust" hard drive.
> >
> I'm curious if this is actually true? (What is it based on?)
> I would think that lifespan would be dependent on I/O
> patterns (because of hardware I/O leveling) -- and filesystem
> vs redo logs could be very different access patterns. In
> particular, redo could easily pound every single block on a
> smaller SSD (hardware leveling becomes fairly meaningless),
> which is rather different from a filesystem where some blocks
> may not get accessed that heavily. I'm not sure one way or
> the other, just something I've been wondering about.
> Similarly, if you mirrored two of them for redo then isn't
> there a high likelyhood that they would wear out around the same time?
> -Jeremy
> --
> Jeremy Schneider
> Chicago, IL
> --
Received on Fri May 01 2009 - 13:33:57 CDT

Original text of this message