Community Matters

…and other systems with cheap and slow SSDs like the Dell Inspiron Mini 9 or Acer Aspire One.

Important note: this is not written for ultrafast SSDs like the Intel SSDs found in high end laptops and servers; it’s targeting the type of SSDs usually found in netbooks. And of course it also doesn’t apply to hard disk based netbooks.

The problem? These very cheap SSDs tend to have terrible write performance. And filesystem designers apparently never considered the impact of devices that have fabulous access times, decent read speed but that come to a screeching halt when sending them too many writes at a time.

Most Linux distributions by default install ext3. Whether or not a journaling filesystem is a good idea for an SSD appears to be still up to debate; but unless the SSD does decent wear-leveling I would stay away from simple journals like what ext3 provides which keep overwriting the same part of the ‘disk’ over and over again.

Also, by default Linux tracks modifications to the access time of every inode. So every read from disk also creates a write updating the atime in the inode. To make things work better on a cheap SSD it’s best to turn this off using the mount option noatime.

The third problem you keep running into is the frequent fsync issue that I’ve described earlier. Here your best bet is a very current Linux distribution that has the latest versions of some of the applications that use fsync more heavily. This is certainly an area where Moblin is trying to be careful.

The fourth problem is the apparently common assumption that a Linux system has to have a swap partition. As it turns out, swap partitions are actually rather counter productive in a system as discussed here. As your system gets more heavily loaded (more things are running and competing for the limited write bandwidth to the SSD) you are now adding the Linux kernel as another creator of write accesses to the SSD – as it is trying to swap out pages to free memory. It’s much better to rely on Linux paging algorithm to keep fewer read only pages in memory – these are read back from “disk” when they are needed again, but as they never get modified they don’t need to be written out to the SSD. Yes, this means that you may run out of memory sooner – but if you do, that may be a sign that you are trying to do things that the little netbook wasn’t designed for (don’t run VirtualBox on it – be careful how many office and email applications you have open at the same time – and yes, limit the number of tabs in Firefox and restart it every once in a while.. Firefox is a real memory hog).

So what do I recommend? Long term it’s BTRFS. I am using this for some tests on one of my EeePCs and the difference is stunning. But BTRFS is far from being stable enough for real life and isn’t (and won’t be for some time) used in any Linux distributions.

In the meantime I’d go with ext2 with noatime and no swap partition. If your system (like most EeePCs) has two separate SSDs with different write speed, make sure that directories that are frequently written to (or fsync’ed) are located on that drive. I have explained how to do this in my posting on solving the Firefox and Evolution fsync issue.

Oh, I’ve played with JFS, XFS and yes, even vfat as possible alternatives. Neither of them provides any noticeable advantage over ext2 – and since they are less frequently used I am concerned about potentially running into bugs and problems that haven’t been discovered by other users, yet.