On 2021/01/27 03:43, Carlos E. R. wrote:
On 27/01/2021 04.11, David C. Rankin wrote:
On 1/26/21 6:37 AM, Carlos E. R. wrote:
On 26/01/2021 01.27, David C. Rankin wrote:
If this is windows on motherboard raid, it's probably dmraid (otherwise known as Fake RAID or BIOS RAID) -- it's not really Fake, it's just the moniker dmraid ended up with from the hardware RAID snobs...
It is really fake, because it doesn't run in hardware: it runs in software, on the computer CPU, with read support on BIOS so that it can boot. Once booted it gets write code from the driver, running on the mainboard CPU, not on the raid chipset.
A true hardware raid doesn't use the mainboard CPU, and is transparent to the operating system.
I'm sure Neil Brown and the rest on the linux-raid list would be surprised to learn it is fake...
It's software... (and the overhead ceased being measurable when 486 came out) Fake is far superior to hardware.
Of course it is :-)
Just have a battery die on your hardware card and drop from write-back to write-through... and then find out your battery was discontinued 3 years ago.
Nothing like people overgeneralizing. 1) Fake Raid -- Since it's not called Fake raid by the OEM's its hard to really say what you are talking about, but Dell ships a BIOS-FIRMware operated RAID though it doesn't operate in all the modes of their HW solutions. But RAID0 and RAID1 are fairly trivial to do, though don't know about combo RAID10(0+1) being supported. Their RAID is supported by pre-OS BIOS HW, so it works with Linux, Windows or whatever. It just looks like an oversized HD to OS's. 2) Whether or not something is better depends on your usage, and the type of RAID you are using. As far as reliability goes, I've had the _dated_ experience of linux kernel crashes back before it was fully SMP and 2 cores weren't as fast as a single same-clock CPU for many peak-speed related tasks, though they were usually able to process more work due to the multitasking nature of most loads. But back in that timeframe, I had the experience more than once (twice) in their 1st year of use of my software RAID5 (linux MD) disks becoming corrupt and unrecoverable before I switched to HW RAID. I had had HWRAID fail once in the following 2 decades due to a "re-manufactured" LSI card that had the heat-sink super-glued on (as I later found out) rather than connected/held using 4 screws with stiff springs + thermal paste as it came new. Problem there, was that I didn't know what a new card was supposed to look like, and I've seen enough motherboards+cards where random chips were epoxied onto the MB with opaque epoxy to prevent reading details from the card or removing the chip in a recoverable fashion to know what was supposed to be spring mounted vs. epoxied-to-prevent tampering. I also found another good difference that made a huge difference in speed between SW+HW raid setups that I used. The SW raids would be very tolerant of diskspeed differences between different disks, but that also meant that striped access didn't measure up to performance. A SW RAID5, with 4 data disks, ran at about the speed of 2-3 single disks in writing & reading. The same disks put in a HW RAID showed that about 9-10 out of 12 were measured as "bad" when attached to a HW RAID card. The reason: they were Deskstars, sold for the home market rather than Ultrastars sold for enterprise. The deskstars varied in speed from the stated 7200 RPMs by as much as 15%, with about 9/12 disks failing due to speed variance. Ultrastars, at the time rand about 33% more for same size, but were within about 1-2% of each other in speed. Second big area of difference -- HW cards can do their own checksumming for RAID5/50/6/60. Beefier cards will have dual cpu's on the RAID card and performed noticeably better on RAID6/RAID60 configs and slightly better than on RAID5/RAID50. Battery backed RAM can allow averaging out write-bursts and higher I/O-ops for greater parallel usage by doing write-back and buffering write-bursts than RAM that is used in write through mode. RAM in the card (or somewhere) is still needed to calculate parity stripes in RAID5+6 modes. It is likely that at least 1 stripe's width is used in card-memory so a full stripe can be written in parallel to each disk. I'm sorta guessing, but RAID0, RAID1 and RAID10 (stripe of mirrors) can keep data in a write buff for the least time since no calculations need be done, but basically, a HW RAID card can abstract out parallel writes from the OS-CPU that can appear to write multiple data disks in the time the OS would normally be able to write 1. Anyway, it hasn't been my experience that SW raid is better than HW raid, but that may be due, in part, to use a common RAID card in my setups (LSI->Avago->Broadcom). FWIW, though 1 PCIe-SSD may well outperform many RAID setups in single-user tasks, with large RAID's using 2.5" disks possibly benefitting needs of DB-users and web-hosting.