04 Mar 2009 @ 3:58  

No SWAP Partition, Journaling Filesystems, … on a SSD?

December 7, 2008

I’m going to get an Asus Eee PC 901go, which has a Solid State Disk (SSD) instead of a normal hard disk (HD). As you know me I’ll remove the installed Linux and install my own Kubuntu. I soon started to look at the best way to install my Kubuntu and I found following recommendations copy and pasted on various sites:

  1. Never choose to use a journaling file system on the SSD partitions
  2. Never use a swap partition on the SSD
  3. Edit your new installation fstab to mount the SSD partitions “noatime”
  4. Never log messages or error log to the SSD

Are they really true or just copy and pasted without knowledge. But first why should that be a problem at all? SSDs have limited write (erase) cycles. Depending on the type of flash-memory cells they will fail after only 10,000 (MLC) or up to 100,000 write cycles for SLC, while high endurance cells may have an endurance of 1–5 million write cycles. Special file systems (e.g. jffs, jffs2, logfs for Linux) or firmware designs can mitigate this problem by spreading writes over the entire device (so-called wear leveling), rather than rewriting files in place. So theoretically there is a problem but what means this in practice?

The experts at storagesearch.com have written an article SSD Myths and Legends – “write endurance” which takes a closer look at this topic. They provide following simple calculation:

  • One SSD, 2 million cycles, 80MB/sec write speed (that are the fastest SSDs on the market), 64GB (entry level for enterprise SSDs – if you get more the life time increases)
  • They assume perfect wear leveling which means they need to fill the disk 2 million times to get to the write endurance limit.
  • 2 million (write endurance) x 64G (capacity) divided by 80M bytes / sec gives the endurance limited life in seconds.
  • That’s a meaningless number – which needs to be divided by seconds in an hour, hours in a day etc etc to give…

The end result is 51 years!

Ok thats for servers, but what is with my Asus 901go?

  • Lets take the benchmark values from eeepc.it which makes it to a max of 50 MByte/sec. But this is a sequential write, which is not the write profile of our atime, swap, journaling… stuff. That are typically 4k Blocks which leads to 2 MByte/sec. (Side node: The EeePC 901go mount the same disk of SSD ‘EeePC S101, to be precise model ASUS SATA JM-chip Samsung S41.)
  • We stay also with the 2 million cycles and assume a 16GB SSD
  • With 50 MByte/sec we get 20 years!
  • With 2 MByte/sec we get 519 years!
  • And even if we reduce the write cycles to 100.000 and write with 2 MByte/sec all the time we’re at 26 years!!

And all this is with writing all the time, even ext3 does write the journal only every 30 secs if no data needs to be written. So the recommendation to safeguard SSDs, as the can not write that often is bullshit!!

So lets take a closer look at the 4 points at the beginning of this blog post.

  1. Never choose to use a journaling file system on the SSD partitions: Bullshit, you’re just risking data security. Stay with ext3.
  2. Never use a swap partition on the SSD: If you’ve enough space on your SSD use a SWAP partition it will not be written onto it until there is to less RAM, in which case you can run a program/perform a task which otherwise you could not. And take a look at this article.
  3. Edit your new installation fstab to mount the SSD partitions “noatime”: That is a good idea if all the programs work with this setting as this will speedup your read performace, specially with many small files. Take also a look at nodiratime.
  4. Never log messages or error log to the SSD. Come on, how many log entries do you get on a netbook? That is not an email server with > 1000 log lines per second.

Please write a comment if you disagree or even agree with my blog post. Thx!

Posted By: stratus
Last Edit: 05 Mar 2009 @ 05:56

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