changing L2ARC drives and SLOG drive

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Ericloewe

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Using a single drive for L2ARC and SLOG presents significant latency issues, which can easily destroy performance.
 

jgreco

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The big thing @tvsjr missed is that a SLOG needs certain characteristics... in particular, it needs to have power loss protection, which all(?) gumstick-based NVMe SSD's lack.

https://forums.freenas.org/index.php?threads/some-insights-into-slog-zil-with-zfs-on-freenas.13633/

The point of the SLOG is to provide a guarantee that data is committed. If you lack the power loss protection (or other form of sync write commit guarantee), then you might as well just go a little further, omit the SLOG, disable sync writes, and get full speed from ZFS.

Installing a SLOG on an SSD without power loss protection is like driving around in a car without your seatbelts because you're convinced the air bags are all that's needed to save you in a crash. It doesn't really work that way.

What this means is that your typical SLOG device is likely to be a relatively expensive "data center" grade device. But also, it doesn't need to be big.

By comparison, what you want for L2ARC is large cheap SSD storage. Preferably two. So what you want to do there is to go out to the local electronics store and see if they have any of last year's SSD's on clearance, you know, the ones that are now basically too small and aren't likely to sell now that 240-512GB sizes are common. If you have a 64GB system, go find yourself a nice pair of 120's for $25 each on clearance (even new, only $38/ea at NewEgg).

As @tvsjr noted, trying to use a single device for both of these introduces contention, and causes you to make perverse choices as to what single device to buy. If you get a suitable device for SLOG with the power loss protection, then you lose out on the L2ARC: either you don't get a large SSD (because a suitable device is very pricey) or you pay a lot (and lose lots of money). Or some people will be convinced that they really don't need the power loss protection, so they're spending money on an SSD that they're just pummeling to death with writes that isn't actually doing for them what the SLOG is supposed to. Like the people who've mistaken a UPS for an SSD's power loss protection feature (probably because they see the words "power loss" and think "means I can duplicate that feature by avoiding power loss"). Those aren't actually anything near equivalent; a cheap SSD that receives a reset command may lose what is in its write cache, for example.
 

Stux

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Was thinking of using an Intel P750 400MB

It's overkill for a SLOG, but has PLP. Would make more sense if I also used it as mostly L2ARC.

It seems like the concern is that a low end SSD would get bottlenecked with the reads/writes, but I'm not sure that is the case with a high end non gumstick ssd.

BUT, I can get 1+GB/s out of two cheap striped sata SSDs, so as you suggest that might be the way to go, at least at first for L2ARC
 

jgreco

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Well, another reason is that FreeNAS doesn't support combined SLOG and L2ARC. You can hack around that with manual ZFS commands, but it really isn't a good idea.

The Intel 750 is a nice choice for a budget SLOG device.
 

doesnotcompute

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removal for the unneeded L2ARC drives went without issue. the 'replace' command/button, where i specify my 200GB s3710 to replace my s3500 for my slog... how long should the "replacing-4" entry be in the volume status window? been 15 min...
 

Ericloewe

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Resilver? It's SLOG, the contents are worthless after a couple of seconds anyway.

I mean, a naïve algorithm could do the swap in less than a minute (assuming sane TXG settings):
  1. Mark SLOG as to be removed, sync writes get directed to on-pool ZIL
  2. Wait for the TXG to be flushed (prod it possibly, to speed things up)
  3. Set the new drive as the ZIL.
 

doesnotcompute

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Resilver? It's SLOG, the contents are worthless after a couple of seconds anyway.

I mean, a naïve algorithm could do the swap in less than a minute (assuming sane TXG settings):
  1. Mark SLOG as to be removed, sync writes get directed to on-pool ZIL
  2. Wait for the TXG to be flushed (prod it possibly, to speed things up)
  3. Set the new drive as the ZIL.

that's what i thought too. but i didn't 'remove' I used the 'replace' command and chose the new drive from the pulldown option as the replacement target.

i was surprised enough that it made me post above.

the 'resliver' wasn't by accident, that's what the warning was (top right - usually green, was yellow saying the volume was reslivering) so i was quoting my gui.

in the end, it took some hours. again, i was surprised, i too thought the slog data was replaced every few seconds. not what i saw using the 'replace' command/button.
 

tvsjr

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What does zpool status show?
 

Stux

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I've noticed it takes a lot longer than you'd (perhaps naively) expect to "replace" a slog, even if the other slog is offlined. Mind you, I think removing/adding is significantly faster.
 
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