Register for the iXsystems Community to get an ad-free experience and exclusive discounts in our eBay Store.
Resource icon

Multiply your problems with SATA Port Multipliers and cheap SATA controllers

In the last year or two, we've had a resurgence of users asking about SATA Port Multipliers and cheap SATA controllers.

Please, do NOT use port multipliers, and use cheap SATA controllers only after extensive research.

SATA controllers and SATA Port Multipliers are some of the cheapest hardware, designed to make a number of hard drives accessible to Windows. A port multiplier takes a single port of a SATA controller and multiplexes it between several drives.

Windows typically only accesses one or two drives at a time, which means that the needs of Windows users are often relatively limited. However, ZFS often accesses data on all drives simultaneously, such as during scrubs and resilvers. This creates a lot of stress on I/O controllers, and tends to expose any problems.

Cheap SATA controllers are often built as PCIe x1 cards, to make them compatible with the largest number of mainboards. On PCIe 2.0, a single PCIe lane is capable of 500MBytes/sec, or at best, two HDD's worth of transfer speed. Yet, many cheap SATA controllers (Syba SI-PEX40064 example) have four SATA ports on them. There is insufficient capacity to talk to all four drives simultaneously.

Worse, often, this is combined with SATA Port Multiplier to offer cards handling as many as 10 drives, so clearly there isn't enough bandwidth there.

So far, all of this assumes that all the hardware works swimmingly well, and even so, it already sucks.

Unfortunately, many cheap SATA controllers are a bit dodgy, and do NOT work well. Because they are price-driven by the PC market to produce the cheapest product, many of these cards use dodgy controllers which may be knockoffs of already poorly-designed controllers by companies who have put out something that is just good enough to work acceptably under Windows. Often, these chipsets do not work particularly well under FreeBSD and Linux, especially when stressed by the massive I/O loads imposed by ZFS.

Worse, SATA Port Multipliers require both a SATA controller that works perfectly, AND a host driver that understands how to control the Port Multiplier correctly. The number of working, trustable combinations of these that exist are relatively few. The fine folks over at BackBlaze and 45Drives built their first generations of storage appliances with these after exhaustive testing that only found a few good combinations. They have posted significant discussion of their experiences over at http://www.45drives.com/blog/uncategorized/our-biggest-design-decision-direct-wired-or-backplane/
which is worth reading before you choose to go down the Port Multiplier path.

Almost all of these "solutions" have been more expensive to buy new than it costs for a used LSI HBA on eBay, where the PERC H200's and PERC H310's are often available for $30, and provide eight fully supported lanes and can use SAS expanders for additional drives beyond that.
Author
jgreco
Views
346
First release
Last update
Rating
0.00 star(s) 0 ratings

More resources from jgreco

Top