I recently migrated from a 314 to 626, which left me with two spare drive bays. As it happens, after upgrading a computer, I also now have two extra Samsung EVO 840 SSDs (750GB, 1TB) lying around.
Now that I've bumped up parts of my LAN to 10G, the biggest bottleneck in the 626 is the HDDs. So I'd like to use the Samsung SSDs in a ReadyTIER setup in my 626 to get maximum performance. My questions are:
Although the ReadyTIER guidelines say the number of SSDs should (not "must") match the number of HDDs, is it possible for me to use 2x SSDs to tier a 4x HDD array?
If so, are there serious (or any) risks in the 2x SSD/4x HDD setup as opposed to an equal 3x setup?
Is a ReadyTIER setup any more prone to irrecoverable data loss than a standard configuration?
Are there any other ReadyTIER caveats that I might be overlooking?
In short, what I'm trying to do is get peak performance from the NAS without creating any additional threat of data loss, and I would love to know what pros and cons to weigh when it comes to using ReadyTIER as a means to that end.
And for the forum folks who love to wag their fingers, yes, the 626 is being backed up daily (to the old 314). But I see restoring data from a backup as a last resort. I don't want to lose the primary data in the first place.
Thanks in advance for any insight or advice!
Model: RN626X|ReadyNAS 626X – 6 Bays with Intel® Xeon® Quad-Core Server Processor
Although the ReadyTIER guidelines say the number of SSDs should (not "must") match the number of HDDs, is it possible for me to use 2x SSDs to tier a 4x HDD array?
Yes, the minimum number of SSDs to start a tier for RAID levels 1 and up would be two. You will be able to get the Add tier button after selecting the two SSDs you have.
If so, are there serious (or any) risks in the 2x SSD/4x HDD setup as opposed to an equal 3x setup?
There shouldn't be any issue or risk on using 2x SSD on your setup but it is always recommended to expand your tier to avoid metadata falling back writing on the mechanical drives. This is if you are using most of your metadata allocated space
Is a ReadyTIER setup any more prone to irrecoverable data loss than a standard configuration?
a ReadyTIER is a RAID group as well, using 2x SSD means you have a RAID1 tier, so you have a 1 disk mirrored. Adding more SSDs for your tier will also change the RAID level.
Are there any other ReadyTIER caveats that I might be overlooking?
Members using the Tier feature would be able to suggest more 🙂
And for the forum folks who love to wag their fingers, yes, the 626 is being backed up daily (to the old 314). But I see restoring data from a backup as a last resort. I don't want to lose the primary data in the first place.
It is always recommended to do backup from several devices or locations so to make sure that data is not lost in the event that we are not expecting to 🙂 using a NAS does not secure data at all times.
Although I'm not using ReadyTier at present, I have kept two slots empty in my RN526 to keep that option open. I'd be interested in hearing details on your performance gains.
On the "caveats" - in thing that occured to me is that since the load on the two SSDs is identical, they will hit their write limits (TBW) at the same time. It might be useful to replace one disk about half way into it's expected life, to prevent that from happening.
Thanks for those detailed answers, @Marc_V. Extremely helpful.
Just to clarify: Let's say @StephenB's scenario comes to pass. Both of my SSDs die, and I lose the metadata. What does that mean for the data stored on my HDDs? It is a simple matter of waiting a few hours for metadata to be regenerated, or do those files become wholly inaccessible?
And how much space does metadata typically take up? Right now I'm using 12TB of 24TB total (bays are 10 | 8 | 8 | 10 | - | - in FlexRaid), and I'd like to know what the chances are that metadata will exceed the volume of the smaller 750GB SSD and start writing to the HDDs.
Knowing those two answers will, I think, help me make a firm decision. I'm really curious to try out ReadyTIER, and I don't mind investing in an brand new 1TB SSD to replace the smaller aging 750GB SSD. But it sounds like the ideal setup would be 3x SSDs, and I'm not keen to lose an HDD in order to create parity with the SSDs. When it comes down to it, I'd rather have more storage volume than more speed.
And how much space does metadata typically take up? Right now I'm using 12TB of 24TB total
My volume is a similar size to yours, and I have 18 GB of space allocated for metadata at the moment. About 12 GB of that is used. So with your SSDs you have more than enough space to do some data caching as well.
Metadata in btrfs is like inodes in ext. So the amount you need will also depend on the number of files you have. The main performance gain you'll get with metadata tiering is browsing folders.
If you log in with ssh, you can see the metadata allocation with
# btrfs fi usage /volume-name
Or just hover your mouse over the pie-chart on the volume wheel.
Just to clarify: Let's say @StephenB's scenario comes to pass. Both of my SSDs die, and I lose the metadata. What does that mean for the data stored on my HDDs? It is a simple matter of waiting a few hours for metadata to be regenerated, or do those files become wholly inaccessible?
The SSD tier isn't a metadata cache. So my understanding is that your data is lost if the SSD array fails. It'd be very helpful to know if that is incorrect.
The SSD tier isn't a metadata cache. So my understanding is that your data is lost if the SSD array fails. It'd be very helpful to know if that is incorrect.
Interesting. I was under the impression that the SSD tier was used exclusively for storing metadata and caching temporary files.
In other words, I was thinking of it as the icing on the cake. So even if the icing were suddenly gone, you'd still have the cake. Maybe someone like @Marc_V or another Netgear mod can clear things up.
EDIT: I looked at the pie chart and saw that I'm only using 8GB in metadata storage with a total of 13GB allocated.
@nsne wrote: Interesting. I was under the impression that the SSD tier was used exclusively for storing metadata and caching temporary files.
Yes, but the metadata is the structures that comprise the file system (e.g., file handles, etc). So without that you just have a pile of raw data blocks.
