DRAM less SSD for ZIL

[rhagu]

Hi,

I have been reading several articles about dram caches on SSDs and that I should look for an SSD with power loss protection or use an optane.
At the moment my server is fairly old and attached with dual gigabit ethernet so not that much to do bandwith wise but I might tweak latency a bit.
Hence my question: If I use two SSDs for ZIL (mirror) shouldn't a setup of SSDs without any DRAM cache at all be just as safe?
The Crucial BX500 series for example.

kind regards,
rhagu

 

[gea]

You want an Slog to protect your rambased writecache on a crash. This is quite senseless if you use an Slog without proper powerloss protection.

Additionally if you want some performance with sync write, you need low latency and high steady write iops and absolutely no desktop SSD.

This means:
The current best of all Slogs is Intel Optane (4801X or without guaranted but very good plp the 800/900 models) or a WD SS530 when you want dualpath SAS.

If they are too expensive, look for a used Intel DC 3700 if you want Sata..

 

[matkisson]

If PLP is something you are looking for check out the (SM953 $60). I just ordered 6 of them from that seller and all had minimal usage (~1800-1900 power on hours, and only <100GB written)

 

[rhagu]

Hi,
thanks for your answers.
@gea : I know all that you said, that is why I wrote: "I have been reading several articles about dram caches on SSDs and that I should look for an SSD with power loss protection or use an optane."
@matkisson: Thanks for the link, I did not know 480GB could be had for such a price

I know performance might tank if I use consumer SSDs, but please keep in mind that my setup is rather old anyway.
My question is really more about understanding the SSDs mechanics. In my mind there should be no power loss problem at all for SSDs that do not use DRAM Cache but use QLC Cells in SLC mode as cache. Because flash is already persistent, right?

 

[BackupProphet]

If you only have dual gigabit, a cheap Intel S3700 100G will do fine.
Another alternative is the 16-32GB Optane module with a pcie->m2 adapter if its an older system. Works fine with PCIe v2 also.

 

[gea]

If you want to write a single Byte to a NVMe/SSD you can do this directly like on RAM with Intel Optane. Traditional flash requires (unless the ssd is new/empty/secure erased/trimmed) to read a whole page (can be several MB), erase the page and write the page newly with the modified Byte.

This is slow without cache. This is why you find dram on most SSDs. A crash during a write cycle can lead to a loss of committed writes. The same can happen during internal optimisation of data (garbage collection). The only type of SSD that are more or less not affected is Optane as there is definitely no dram needed or used.

Remain the problem that a write on flash is quite complex. Not only the dram can be the reason of a dataloss on a crash during write. Firmware is as important.

No it comes to powerloss protection. There are several stages as the term "plp" can be used by anyone to describe different things.

1. The reliable ones
This is a guaranteed feature of trusted datacenter disks from Intel, Micron or Samsung ex Intel DC SSD or Optane 480x.

2. The possibly trusted one
Although not guaranteed, all Optane should have an at least good plp behaviour. Intel guarantees only with the datacenter line 480x not the others.

3. Vendor declares "plp" but this covers only some aspects. (Powerloss immunity or powerloss prevention). More a marketing aspect than a serious guarantee.

4. all the rest.
They do not care about.

 

[MrCalvin]

You want an Slog to protect your rambased writecache on a crash. This is quite senseless if you use an Slog without proper powerloss protection.

I second that statement. PLP's primary use it NOT protecting your data, a PLP-less drive is just as safe when speaking synchronous writing, which I assume is the case with ZIL.
A PLP-less drive should not reply back with an "OK, data is now written to disk" until it is actually is written to the disk and not just the DRAM cache.
But a PLP drive give you a performance advantage because it actually can say OK when data is just in the cache!
That's why it is so important to look at the PLP protected cache size, not just to see if the disk has a PLP label. If the cache is only 10KB the PLP label on the disk is nothing more than a marketing gimmick only the novice user will be intrigued by
An interesting question would be: Would a SSD SATA with large PLP protected cache give you more performance than a NVMe with small PLP protected cache, I might very well be so!

Unfortunately the PLP protected-cache size if very rarely (as in never) specified by the HDD manufacturers, not even when you ask them. The only one I could get this information from was Kingston, neither Seagate or Micron would reveal those data, it makes you wonder why!?
And unfortunately most (as in all) HDD reviewers don't ask the question either :-(
See this thread "Power-Loss-protected cache size on SSD: Yes, finally someone reveals the size (Kingston)"

 

[i386]

Would a SSD SATA with large PLP protected cache give you more performance than a NVMe with small PLP protected cache

PCIE 3.0 x4 > Sata

Simplified explanation with numbers:
host/sata controller <- 6GBit/s ~600MByte/s -> sata device/nand controller <- 12800 MByte/s -> ddr3-1600 cache
host/cpu <- 4x 8GT/s ~3.9GByte/s -> pcie ssd/nand controller <- 12800 MByte/s -> ddr3-1600 cache

DDR3-1600 = 12800 MByte/s

The pcie/nvme ssd could fill up a smaller cache multiple time per second, a sata ssd will take ~1.7 seconds to fill up 1GByte cache