Mageia 3 or 4 and hybrid disks

[fonon]

Hi,
I have two questions about hybrid disks:
1) How Mageia works with hybrid disk (boot drive) in single operating system configuration? Any problems?
2) How the hybrid disk works in multi-boot configuration (2 or more OS, Windows + Linux configuration)?
The semiconductor memory of 8GB probably works fine with single operating system, but is it capable speed up more operating systems installed on this disk?
f.

[doktor5000]

What hybrid disks are you talking about exactly? You mean SSHDs - HDDs with SSD/flash cache?
Except for the WD Black SSHD (aka Black² Dual Drive) all SSHDs behave as normal harddrives and those are transparent for the operating system - you don't have to care about them.

[fonon]

What hybrid disks are you talking about exactly? You mean SSHDs - HDDs with SSD/flash cache?

Yes.

Except for the WD Black SSHD (aka Black² Dual Drive) all SSHDs behave as normal harddrives and those are transparent for the operating system - you don't have to care about them.

This is the answer to my first question. The hybrid disk has very limited (e.g. 8GB) semiconductor memory. It's enough for single operating system. The benefit of hybrid disk is the frequently used data is stored in this part of HDD memory. When the disk works in multi-system environment the flash memory must be shared by different operating systems. Does it work?
f.

[ghmitch]

I think your question is rather difficult to answer because a lot depends on what you mean by "work". First of all, stay away from that Western Digital drive. It is software managed making it pretty much a Windows only drive. Second, as doktor has explained already, the others operate just like any other drive. The only difference is the inclusion of the flash drive based "super cache" integrated into the conventional drive. The drive firmware uses this enhanced cache space to profile and store the most commonly accessed data, similarly to what Linux itself does in RAM with "drive cache" on the host side. So, theoretically, this should result in a "faster" drive. In a multiboot state, each component OS should benefit to the degree that it is used. If you mostly use Linux, for example, and Windows only occasionally, Linux would likely get the lion's share of the flash cache simply because it accesses the drive more than the Windows side. If you use Windows more, the opposite would likely be true. But in the end they would share the available flash space, but not equally. These devices DO have a reputation for speeding up performance, but by how much is probably somewhat subjective and dependent on how you as an individual use your computer.

Ideally, there will be someone here who has acutally used one of these drives in this sort of configuration who can comment more directly on the subject as I, myself, have not. But they appear to the computer hardware and OS just as any other drive has.

Here is a link to Seagate's description of their hybrid drive product and how it works: http://www.seagate.com/files/www-content/product-content/momentus-fam/momentus-xt/en-us/docs/mb618-solid-state-hybrid-drive-us.pdf

[fonon]

Thank You for answer and explanation. I know, there is no simple answer to my question. I understand that the flash memory may be shared by multiple operating systems located on separate partitions with different file systems and the only criterion of moving data to the "super cache" is the frequency of data reading or writing. Could You estimate the disk space (system files size) used by Mageia in system start/shutdown cycle for simple desktop - office configuration (excluding applications like word processors and their own data for example)? I suppose, this space in semiconductor memory is needed to benefit from hybrid disk extra speed (if the application files and data aren't significant size). If OS shares 4GB of HD space for example, and the size of "super cache" is 8GB, I can install 2 copies of system and both will work fine.
f.

[doktor5000]

I think you're totally over-complicating this needlessly. Only one operating system will access that disk at one time, the cache will only be used by that operating system.
You cannot install directly on the flash area, the disk will automagically cache frequently-acessed areas. Simply install like you would normally do and don't think about it.
That's it.

Please mark the thread accordingly by editing the topic of the first post and prefix it by [DONE], thanks

[fonon]

I think you're totally over-complicating this needlessly.

You don't understand the problem. The disk will work with any number of OS installed - it's clear. But each OS (working) inserts his own data into nand memory and push out the data belonging to other systems. Next time, when other OS starts, his files isn't present in the flash memory. This effect may cancel the benefits of hybrid disk.
f.

[doktor5000]

If you care so much about performance, get an SSD, simple as that.

If you're thinking about trying to only have 2-4GB per operating system get cached in the flash memory, you end up with a lot of problems if you're serious about that:
- what about all other disc accesses not related to bootup and all normal disk accesses on that hybrid disk? Those will invalidate your precious bootup cache
- how do you want to control that?
- how do you measure if there's a performance impact if the cache is not primed with the OS stuff you care about? how do you plan to fix that?
- if you're investing that much efforts into this, this would already warrant the additional expenses for an SSD

[fonon]

If you care so much about performance, get an SSD, simple as that.

Unfortunately, they are very expensive, especially - high capacity SSD.

If you're thinking about trying to only have 2-4GB per operating system get cached in the flash memory, you end up with a lot of problems if you're serious about that:
- what about all other disc accesses not related to bootup and all normal disk accesses on that hybrid disk? Those will invalidate your precious bootup cache

Simply, I'm frequently using only small files as user data while very large files - not often (but I need to have it on disk, so I need a disk space for large files).

- how do you want to control that?

I don't need to control that.

- how do you measure if there's a performance impact if the cache is not primed with the OS stuff you care about? how do you plan to fix that?

I don't need to measure anything. It's a practical problem only.

- if you're investing that much efforts into this, this would already warrant the additional expenses for an SSD

What efforts? I'm simply asking.
The right comment was made previously by ghmitch:

(...) Ideally, there will be someone here who has acutally used one of these drives in this sort of configuration who can comment more directly on the subject as I, myself, have not.

f.

[ghmitch]

Just to clarify how the Seagate drives work. Frequency of use determines what is stored in cache without regard to OS. Thus the OS you use most will end up with the most files stored in cache. What is stored in cache is determined by drive firmware, not OS software, so on drive cache storage is determined only by the drive itself. This cache is NOT reinitiated with each new boot, it is, rather, persistent in nature. Only changes in usage over time will modify its content. Additionally, this is TRUE cache. Everything stored on it will also be mirrored on the hard drive to protect against failure at the flash level. Thus nothing is MOVED from the hard drive into flash cache. Rather frequently accessed files are COPIED from the drive to the cache and on read requests, the cache is always checked first and proceeds to hard disk if the data in question does not reside in cache.

Essentially what doktor is trying to tell you is true. For serious speed the only solution is SSD. But with either single OS or multi boot, the hybrid drive should result in increased speed. But since everyone uses their computer differently, it is difficult to make any where near precise predictions. In other words, you will get mileage out of this, but it WILL vary depending on your unique usage environment. SSDs are coming down in price, but they are still pricey compared to conventional drives. So if you want speed without the extra cost, the only solutions are hybrid drive or software RAID0 and hybrid drive is less complex and more reliable than software RAID0. And it will likely focus the speed where you need it most.

Additionally, I don't think that your question is specific to Mageia, so you might want to ask this same question on a larger generic Linux forum phrasing the question as "Multiboot performance on hybrid drives?" or something of that sort. I had hoped that you would get an answer here, but this gets into an area that is a bit specialized so maybe not. But the question would definately benefit from a larger audience. I use https://www.facebook.com/groups/GNUAndLinux/. With almost 80,000 Linux users as members, it is very diverse.

[doktor5000]

and on read requests, the cache is always checked first and proceeds to hard disk if the data in question does not reside in cache.

Small addition: From what I've read the cache is only checked on read requests, writing goes straight to the harddisk ignoring the cache, which makes sense.