If you are a customer or potential customer for Arista data center switches, you should get a copy of Arista Warrier by Gary Donahue. Published October 2012.
The 7124SX uses Intel's Ethernet Switch FM4000, introduced August 2006. Intel's data sheet has the following diagram:
The Shared Memory Fabric is partially dedicated to each port with the remainder in a dynamic pool. In our summary table, 1.238 + 0.02 MB is the dynamic pool + fixed port allocation. The sum is the largest buffer that is available to a single flow -- assuming the dynamic pool is not otherwise engaged by traffic queued on other ports.
Some Arista 7124SX information is from The Open Industry Network Performance & Power Test for Cloud Networks Evaluating 10/40 GbE Switches Spring 2013 Edition dated July 2013. Quoting a single sentence:
"The Arista 7124SX has a 2MB packet buffer pool and uses Dynamic Buffer Allocation (DBA) to manage congestion." n.b. The quote is from the Lippis Report and not from Arista. Arista's claim is that there is 2 MB of RAM and 1.7 MB of this is available to be used as packet buffers. Arista published a short microburst white paper that backs this up.
See page 29. Get this from http://lippisreport.com. Requires registration.
Info on the 7148SX comes from an Arista rant expressing displeasure at how sleezey the test-for-hire industry is. Arista seems to want to define microburst in terms relating specifically to the financial services industry. A broader definition is a clump of packets big enough to run out buffers but not so big as to be visible in polled SNMP stats. I believe that microburst tolerance is a fair characterization of a feature that is useful in big-data-high-latency file transfer applications. But the rest of the rant seems spot on.
The 7148SX uses four FM4000s to double the port count and provide chip-to-chip interconnects. A packet that routes through a single FM4000 has access only to one chip's dynamic pool. A packet that routes through an interconnect may be able to access the dynamic pools along its path -- if there is flow control in the interconnects.
End of Life for the 7124SX and 7148SX has been announced. The 7150S-xx replacement switches will use a newer Intel/Fulcrum switch -- the FM6000. n.b. The Warrior book was written before the announcement of the replacement switches.