Which RAID for read/write cache on Qnap?
I can except I cannot allocate SSD cache independently on 2 – apologies but start at the bottom screen shot and go up.
I go through what happens for 1 ssd and then what happens for 2.
Hope it helps otherwise I need to go read write on both and if so do I use raid 1 or 0 as I’m not planning on using it for storage
Thank you for coffees again. Helps my coffee addiction 🙂
You will need to choose Read/Write option. This ways system will figure out how to use one for read and other for write automatically theoretically.
Most importantly you have to do this otherwise setting single SSD for write cache would lead to possible data loss in the future. With write cache enabled all written data first is stored on SSD and later moved to HDD. If this SSD fails, your data would be lost. For that reason Qnap suggest to have two SSD for write cache. If you would want to keep write and read cache separate you would need 3 SSD. Two SSD in RAID1 for write and single (or again dual SSD RAID1/RAID0) for read cache.
Combining read/write ends up cheaper without effecting performance.
RAID0 for cache is only recommended for read cache, since all data still lives on HDDs while copy is created on SSD. But not with data writing.
RAID1 will mirror both SSD and system will split them in half to perform read/write cache operations.
I hope this helps.
Creating the SSD Cache
- Go to .
- Click .
The Create SSD Cache window opens.
- Click Next.
- Select one or more SSDs.
All data on the selected disks will be deleted.
- Select a cache type.
When data is read from a LUN or volume, QTS copies the data to the SSD cache to speed up future read requests.
QTS writes incoming data to the SSD cache first, then flushes the data to regular storage later. Read access to the new data is also accelerated while it is in the cache.
QTS uses the SSD cache for both read and write caching, accelerating both read and write speeds.
- Select a RAID type.
Selecting a RAID type with no disk failure protection (Single, JBOD, RAID 0) when the cache type is Write-only or Read-write may result in data loss.Tip:
RAID 10 provides the best write cache performance.
- Click Next.
- Optional: Configure SSD over-provisioning.
Over-provisioning reserves a percentage of SSD storage space on each disk in the RAID group to improve write performance and extend the disk’s lifespan. You can decrease the amount of space reserved for over-provisioning after QTS has created the RAID group.Tip:
To determine the optimal amount of over-provisioning for your SSDs, download and run SSD Profiling Tool from App Center.
For details, see SSD Profiling Tool.
- Select a cache mode.
Recommended Use Cases
Only small data blocks are added to the SSD cache. Larger blocks are accessed directly from regular storage.
Small and large data blocks are added to the SSD cache. Both sequential and random I/O requests are accelerated.
Video streaming, large file access operationsTip:
An HDD RAID group may outperform a SSD RAID group for sequential I/O if the ratio of HDDs to SSDs is 3:1 or greater, and the HDD group has a RAID type of RAID 0, 5, 6, or 10. However, SSDs will always be faster for random I/O. If the NAS contains a RAID group of type RAID 0, 5, 6, or 10 that contains three times more disks than the SSD cache, you should select Random I/O.
- Optional: Configure the following advanced settings.
Bypass block size
This value determines the maximum size of the data blocks that are stored in the SSD cache. Selecting a larger size may improve the cache’s hit rate but uses more cache space. The default value is 1 MB.
Cache replacement policy
Specify how data is removed from the SSD cache. Choose one of the following options:
Least recently used (LRU): Better cache performance but uses more CPU resources. This is the default option.
First in first out (FIFO): Lower CPU usage than LRU but might cause worse cache performance.
- Click Next.
- Select which volumes and LUNs can use the SSD cache.
- Click Next.
- Click Create.
A confirmation message appears.
- Select I understand and then click OK.