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Forum Discussion
Basic ReadyTIER Questions
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. ...
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.
nsne wrote:
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.
nsne wrote:
But it sounds like the ideal setup would be 3x SSDs,
Not sure why you are thinking that, personally I think RAID-1 is probably better than RAID-5 for the SSD Tier.
nsne wrote:
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?
That's an important question, and the guides don't say. https://www.downloads.netgear.com/files/GDC/READYNAS-100/ReadyNAS_FlexRAID_Optimization_Guide.pdf
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.
StephenB wrote: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.
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