Ditching the spinning rust

For some time now I’ve been thinking of switching my laptop storage over to an SSD. I like the idea of the massively improved performance, the slightly reduced power consumption, and the ability to better withstand the abuse of commuting. However, I don’t like the limited write cycles, or (since I need a reasonable size drive to hold all the data I’ve accumulated over the years) the massive price-premium over traditional drives. So I’ve been playing a waiting game over the last couple of years, and watching the technology develop.

But as the January sales started, I noticed the prices of 256GB SSDs have dipped to the point where I’m happy to “invest”. So I’ve picked up a Samsung 840 EVO 250GB SSD for my X201 Thinkpad; it’s essentially a mid-range SSD at a budget price-point, and should transform my laptops performance.

SSD’s are very different beasts from traditional hard drives, and from reading around the Internet there appear to be several things that I should take into account if I want to obtain and then maintain the best performance from it. Predominant amongst these are ensuring the correct alignment of partitions on the SSD, ensuring proper support for the Trim command, and selecting the best file system for my needs.

But this laptop is supplied to me by my employer, and must have full system encryption implemented on it. I can achieve this using a combination of LUKS and LVM, but it complicates the implementation of things like Trim support. The disk is divided into a minimal unencrypted boot partition with the majority of the space turned into a LUKS-encrypted block device. That is then used to create an LVM logical volume, from which are allocated the partitions for the actual Linux install.

Clearly once I started looking at partition alignment and different filesystem types a reinstall becomes the simplest option, and the need for Trim support predicates fairly recent versions of LUKS and LVM, driving me to a more recent distribution than my current Mint 14.1, which is getting rather old now. This gives me the opportunity to upgrade and fine-tune my install to better suit the new SSD. I did consider moving to the latest Mint 16, but my experiences with Mint have been quite mixed. I like their desktop environment very much, but am much less pleased with other aspects of the distribution, so I think I’ll switch back to the latest Ubuntu, but using the Cinnamon desktop environment from Mint; the best of all worlds for me.

Partition alignment

This article describes why there is a problem with modern drives that use 4k sectors internally, but represent themselves as having a 512byte sector externally. The problem is actually magnified with SSD’s where this can cause significant issues with excessive wearing of the cells. Worse still, modern SSDs like my Samsung write in 4K pages, but erase in 1M blocks of 256 pages. It means that partitions need to be aligned not to “just” 4K boundries, but to 1MB boundries.

Fortunately this is trivial in a modern Linux distribution; we partition the target drive with a GPT scheme using gdisk; on a new blank disk it will automatically align the partitions to 2048 sector, or 1MB boundries. On disks with existing partitions this can be enabled with the “l 2048” command in the advanced sub-menu, which will force alignment of newly created partitions on 1MB boundries.

Trim support

In the context of SSD’s TRIM is an ATA command that allows the operating system to tell the SSD which sectors are no longer in use, and so can be cleared, ready for rapid reuse. Wikipedia has some good information on it here. The key in my case is going to be to enable the filesystem to issue TRIM commands, and then enabling the LVM and LUKS containers that hold the filesystem to pass the TRIM commands on through to the actual SSD. There is more information on how to achieve this here.

However, there are significant questions over whether it is best to enable TRIM on the fstab options, getting the filesystem to issue TRIM commands automatically as it deletes sectors, or periodically running the user space command fstrim using something like a cron job or an init script. Both approaches still have scenarios that could result in significant performance degradation. At the moment I’m tending towards using fstrim in some fashion, but I need to do more research before making a final decision on this.

File system choice

Fundamentally I need a filesystem that supports the TRIM command – not all do. But beyond that I would expect any filesystem to perform better on an SSD than it does on a hard drive, which is good.

However, as you would expect, different filesystems have different strengths and weaknesses so by knowing my typical usage patterns I can select the “best” of the available filesystems for my system. And interestingly, according to these benchmarks, the LUKS/LVM containers that I will be forced to use can have a much more significant affect on some filesystems (particularly the almost default ext4) than others.

So based on my reading of these benchmarks and the type of use that I typically make of my machine, my current thought is to run an Ubuntu 13.10 install on BTRFS filesystems with lzo compression for both my root and home partitions, both hosted in a single LUKS/LVM container. My boot partition will be a totally separate ext3 partition.

The slight concern with that choice is that BTRFS is still considered “beta” code, and still under heavy development. It is not currently the default on any major distribution, but it is offered as an installation choice on almost all. The advanced management capabilities such as on-the-fly compression, de-duplication, snapshots etc make it very attractive though, and ultimately unless people like me do adopt it, it will never become the default filesystem.

I’ll be implementing a robust backup plan though!


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