Recent Articles

ZFS dedup: tested, found wanting

Published by Jim Salter // February 24th, 2015

Even if you have the RAM for it (and we’re talking a good 6GB or so per TB of storage), ZFS deduplication is, unfortunately, almost certainly a lose. I don’t usually have that much RAM to spare, but one server has 192GB of RAM and only a few terabytes of storage – and it stores a lot of VM images, with obvious serious block-level duplication between images. Dedup shows at 1.35+ on all the datasets, […]

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Exploring Copyleft

Published by Jim Salter // February 11th, 2015

Recently, the ElementaryOS Linux distribution screwed the pooch pretty badly, PR-wise – TL;DR if ElementaryOS nerfs the blog post they never should have made: “if you don’t donate money when you download ElementaryOS, you’re a cheater”. Yeah, that didn’t go down so well. Where it got interesting was this Reddit thread discussing the gaffe. It never ceases to amaze me what a slippery concept copyleft really is, and how easy it is for people to […]

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ZFS: You should use mirror vdevs, not RAIDZ.

Published by Jim Salter // February 6th, 2015

Continuing this week’s “making an article so I don’t have to keep typing it” ZFS series… here’s why you should stop using RAIDZ, and start using mirror vdevs instead. The basics of pool topology A pool is a collection of vdevs. Vdevs can be any of the following (and more, but we’re keeping this relatively simple): single disks (think RAID0) redundant vdevs (aka mirrors – think RAID1) parity vdevs (aka stripes – think RAID5/RAID6/RAID7, aka […]

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Will ZFS and non-ECC RAM kill your data?

Published by Jim Salter // February 3rd, 2015

This comes up far too often, so rather than continuing to explain it over and over again, I’m going to try to do a really good job of it once and link to it here. What’s ECC RAM? Is it a good idea? The ECC stands for Error Correcting Checksum. In a nutshell, ECC RAM is a special kind of server-grade memory that can detect and repair some of the most common kinds of in-memory […]

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Why Sanoid’s ZFS replication matters

Published by Jim Salter // November 19th, 2014

If you’re an old hand in the storage game and are familiar with rsync (which is an amazing tool, btw) you might not be quite sure why block-level replication matters. So, let’s do a thought experiment What if you want daily offsite DR of an entire VM image? Let’s say the image is about 2TB in size. If you just copy the whole thing, like with FTP or SCP or any other simple copy tool, […]

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  • Recent Thoughts

  • Demonstrating ZFS pool write distribution
  • One of my pet peeves is people talking about zfs “striping” writes across a pool. It doesn’t help any that zfs core developers use this terminology too – but it’s sloppy and not really correct. ZFS distributes writes among all the vdevs in a pool.  If your vdevs all have the same amount of free space available, this will resemble a simple striping action closely enough.  But if you have different amounts of free space on different […]