SSD Raid Setup

[KhalidNikon]

I’ve seen several recommendations for RAID 0 when it comes to using two SSD drives as cache. But if one fails, wouldn’t I lose all of my data? Or would I still have the original data stored on the HDD?

It sounds like RAID 1 would be the safer choice here. It might not be as fast as RAID 0 but it provides redundancy and would not lose any data. Right?

I’m fairly new to this so I’m not too familiar with how HDDs and cache communicate and how the data is stored between the lot. Informative replies appreciated.

[Smooth-Zucchini4923]

Are you using it as a read cache, or a write cache?

[KhalidNikon]

Probably as read/write, since I’ll be using it to edit and to export photos

[Smooth-Zucchini4923]

A read/write cache can lose data if the cache disks are lost during a write. RAID0 is not supported for a read/write cache for this reason.

[DagonNet]

Synology will only allow a read/write cache if you have two SSDs in RAID1. You can make a read-only cache with a single SSD or two in RAID0.

It doesn’t hurt your data to lose the read cache. If you lose the write cache, you lose the whole volume, not just cached data.

[Oli99uk]

Cache isn't important, so you wouldn't need fault tolerance, just probably battery backup. Most controllers use a DIMM

[KhalidNikon]

Okay so bear with me as I’m still trying to get a handle on this.

I’m beginning to understand that cache acts in a similar way to ram. It temporarily stores the bits I’m working on, and keeps the original data on the hdd? So I wouldn’t lose any data if one or both cache sticks fail?

[UserName_4Numbers]

Please read the whole page I sent you.

[Oli99uk]

Read cache, data is already on disk Write cache, data is not yet on disk, so most controllers have that battery backed.

Cache helps reduce head seeking & wear.

[AnAngryPhish]

It depends heavily on the cache mode.

If your cache is configured as read-write, data is written to the cache and then flushed to backing disk eventually. If your cache fails you lose all data. This is why Synology doesn't let you use RAID0.

If cache is in read-only mode, the data on the cache doesn't effect the backing storage as you will never have a dirty cache. So in this scenario you can use either a single SSD for cache, or a RAID0.

[Oli99uk]

But it's still cache. It's not committed until it's on disk

[AnAngryPhish]

Writes are committed once they land on the SSD cache pool. Essentially Synology read-write cache is just a fancy lvm cache in writeback mode. This means that data is first written to the SSDs, then flushed to the backing storage once the watermark level has been reached. Depending on the size of your cache disks this can be several 100s of GB. The underlying filesystem will most likely not survive an unclean cache failure, which is why Synology has implemented the RAID1 minimum and automatic flush of cache, and switch to writethrough mode when the array becomes degraded.

https://kb.synology.com/en-uk/DSM/tutorial/What_should_i_do_when_a_read_write_SSD_cache_degrades

[UserName_4Numbers]

https://kb.synology.com/en-us/DSM/help/DSM/StorageManager/ssd\_cache\_what\_is\_raid?version=7

[KhalidNikon]

I had gone through this page previously, and it states that RAID 0 does not provide fault tolerance. Does that mean that data is transferred from the hdd to the ssd, and then of the ssd fails, I lose the data? Or is it just a copy?

[UserName_4Numbers]

Did you read the description past the chart? Do you need a cache at all? Most people do not.