Monthly Archives: August 2014

LVM, TLC NAND and the power of 2 problem

I recently bought a Samsung Evo 840 1TB SSD drive. These drives perform pretty good for consumer drives, and despite the lack of endurance figures, some user tests show that it lasts just fine; 80TB isn’t in the enterprise range but it’s sufficient for consumer grade, e.g. desktops and laptops. Also, the larger the drive, the  longer it can last, provided you aren’t filling it up completely.

With a drive this big, LVM is very practical; I tend to rearrange storage allocation quite a bit over time, and virt-manager does a good job of managing LVM as a disk backend for VMs. Generally using raw disks or logical volumes is also a bit faster for your VMs, though you lose some nice features of the qcow2 and other file container formats.

As we know by now, aligning your SSD properly is important if you care about performance – but equally so for its lifetime. Proper alignment on erase block sizes ensures minimal P/E cycles which will make your drive last longer.. But there is a problem with TLC drives that doesn’t affect SLC or MLC – the erase block size is not a power of 2. It’s 1536kb for TLC, whereas SLC and MLC are 512kb and 1024kb or multiples of that.

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