write up blob storage (w/ seaweedfs)

This follows-on to Martin's earlier proposal.

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diff --git a/blobs/README.md b/blobs/README.md
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+
+This document describes sandcrawler/fatcat use of "blob store" infrastructure
+for storing hundreds of millions of small files. For example, GROBID XML
+documents, jpeg thumbnails of PDFs.
+
+The basic feature requirements for this system are:
+
+- don't need preservation data resiliency: all this data is derived from
+  primary content, and is usually redundantly stored in Kafka topics (and thus
+  can be re-indexed to any server bounded only by throughput of the object
+  store service; Kafka is usually faster)
+- don't require SSDs or large amounts of RAM. Ability to accelerate performance
+  with additional RAM or moving indexes to SSD is nice, but we will be using
+  spinning disks for primary data storage
+- hundreds of millions or billions of objects, fetchable by a key we define
+- optional transparent compression (for text and XML)
+- typical object (file) size of 5-200 KBytes uncompressed, want to support up
+  to several MBytes
+- very simple internal API for GET/PUT (S3 API compatible is good)
+- ability to proxy to HTTP publicly for reads (eg, HTTP fall-back with no
+  authenticaiton), controllable by at least bucket granularity
+
+## Infrastructure
+
+`minio` was used initially, but did not scale well in number of files. We
+currently use seaweedfs. Any S3-compatible key/value store should work in
+theory. openlibrary.org has used WARCs in petabox items in the past. Actual
+cloud object stores tend to be expensive for this kind of use case.
+
+The facebook "haystack" project (and whitepaper) are good background reading
+describing one type of system that works well for this application.
+
+
+## Bucket / Folder Structure
+
+Currently we run everything off a single server, with no redundancy. There is
+no QA/prod distinction.
+
+Setting access control and doing bulk deletions is easiest at the bucket level,
+less easy at the folder level, most difficult at the suffix (file extention)
+level.
+
+For files that are derived from PDFs, we use the SHA-1 (in lower-case hex) of
+the source PDF to contruct keys. We generate nested "directories" from the hash
+to limit the number of keys per "directory" (even though in S3/seaweedfs there
+are no actual directories involved). The structure looks like:
+
+    <bucket>/<folder>/<byte0>/<byte1>/<sha1hex><suffix>
+
+Eg:
+
+    sandcrawler/grobid/1a/64/1a6462a925a9767b797fe6085093b6aa9f27f523.tei.xml
+
+The nesting is sort of a hold-over from minio (where files were actually
+on-disk), but seems worth keeping in case we end up switching storage systems
+in the future.
+
+## Existing Content
+
+sandcrawler: internal/controlled access to PDF derivatives
+    grobid: TEI-XML documents
+        extension: .tei.xml
+    text: raw pdftotext (or other text transform)
+        extension: .txt
+
+thumbnail: public bucket for thumbnail images
+    pdf: thumbnails from PDF files
+        extension: .180px.jpg
+
+## Proxy and URLs
+
+Internal HTTP access via:
+
+    http://wbgrp-svc169.us.archive.org:8333/<bucket>/<key>
+
+Public access via:
+
+    https://blobs.fatcat.wiki/<bucket>/<key>
+
+Eg:
+
+    http://wbgrp-svc169.us.archive.org:8333/testing/small.txt
+    http://wbgrp-svc169.us.archive.org:8333/sandcrawler/grobid/1a/64/1a6462a925a9767b797fe6085093b6aa9f27f523.tei.xml
+    https://blobs.fatcat.wiki/testing/small.txt
+    https://blobs.fatcat.wiki/thumbnail/pdf/1a/64/1a6462a925a9767b797fe6085093b6aa9f27f523.180px.jpg
+
diff --git a/blobs/seaweedfs/README.md b/blobs/seaweedfs/README.md
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+
+## HOWTO: Create new bucket in SeaweedFS
+
+Log in to the seaweedfs VM.
+
+Run `weed shell` to start a shell, then:
+
+    bucket.create -name <bucket>
+