Hacker News Reader

Comment by megous | Hacker News Reader

A2 needs support in the Linux kernel. It's not there, yet.

https://github.com/ThomasKaiser/Knowledge/blob/master/articl...

"A2 promises even better performance with 4000/2000 read/write IOPS minimum but there's a problem since as outlined by the SD Association A2 cards show "much higher performance than A1 performance by using functions of Command Queuing and Cache".

Cache and Command Queuing require host (driver) support since the host needs to activate those new features first. The cache feature on A2 rated cards makes use of volatile RAM on the card requiring the host to learn new commands to issue flushing the cache (involving the risk of data losses -- for details see especially chapter 4.17 in Physical Layer Simplified Specification 6.0)"

View on HN

So is there any kind of device able to use A2 cards at the rated performance yet? I can't get anything better on my Mac with multiple readers (USB 3.0 and 3.1), and I'm guessing if I tried on the reader in my 1 year old Dell XPS, I'd get the same.

What good is a performance designation (A2) if no device in the real world supports it?

I don't know. It's a bit of a bummer.

I imagine Android would benefit the most (and phones also don't suffer from sudden power outages, which is the biggest risk with enabling this), so someone may try looking if mmc core in androind kernel has patches to support this.

My phone will reboot randomly; depending on what layer the reboot happens at, there may be no flush command sent to the card.

No worries, I checked it a few minutes ago, and android doesn't support this either.

> and phones also don't suffer from sudden power outages

Perhaps you have never dropped your phone, but in my experience, on a dropped phone the battery cover and the battery often go flying.

I will say that at least most flagship phones these days have internal batteries.

Flagships have been skyrocketing in price and are losing marketshare to the midrange segment, which almost always has removable storage, and often has a swappable battery.

> and phones also don't suffer from sudden power outages,

Phone batteries die, and the battery remaining percentages are only an estimate so writing code to react to that condition can be fickle.

> What good is a performance designation (A2) if no device in the real world supports it?

this is often the case for new hardware specs/protocols..

see also: any SATA/SCSI rev N+1, TCQ/NCQ, UASP, PCI n+1, etc.

But in all those cases (I have used most of them), there was _some_ way a consumer could acquire hardware (and drivers) to use the tech. It might've been expensive, but I could go out and buy an SATA card, or SCSI 2, or whatever.

With A2, so far I have not seen any way any consumer (or anyone, really) could achieve 2000/4000 IOPS.

That would be like selling a SATA 2400 drive for 2x the cost of an SATA 600 drive... but not selling any kind of controller that could use more than SATA 600 bandwidth.

It should be possible to patch the kernel. If I had an A2 card, I'd try. It's probably just a question of detecting card features on card plug-in and sending a command to enable the cache.

Spec/code is open. I'm just sort of surprised noone tried it yet.

Assuming the cards with the A2 designation do actually support the claimed speeds, support will (eventually) be added, and the faster performance will be observed, on the Pi and other host devices.

You will probably need a true SD/MMC controller to use any "new" commands, such as a smartphone or embedded target (such as the RPi). USB or other bridges used on notebooks are unlikely to have support for it (especially as they convert MMC/SD to a USB-storage class device)

Does anyone know if an improved Linux driver is in the works?

Reply on news.ycombinator.com